BTA CBDE Certification Sample Questions

02. A function contains this loop, where total is a state variable and items is a memory array:
for (uint i = 0; i < items.length; i++) { total += items[i]; }
Which change lowers the gas cost while producing the same final value of total?
a) Mark the function view so the loop's reads and writes are not charged
b) Declare total as constant so updating it inside the loop is free
c) Move the loop into a second contract and call it externally, so its storage writes are billed to that contract instead
d) Sum into a local variable, then write total to storage just once at the end

03. Users of a protocol worry that the admin could push a malicious upgrade and immediately take their deposits.
Which control most directly gives users a chance to leave before an admin change takes effect?
a) A timelock that holds each queued admin action for a publicly visible waiting period
b) A multi-signature admin wallet so that no single keyholder can push the change
c) Publishing the logic contract's full source code and ABI on the project website for review
d) An emergency stop that the admin can activate whenever users report suspicious behavior to the team

04. A team wants its contract to have a known address before it is deployed, one that stays the same even if the deployer account sends other transactions in the meantime. It deploys through a factory contract that uses the CREATE2 opcode.
Which inputs determine the resulting contract address?
a) The creation code alone, so every factory would produce the identical address
b) The deployer account's address and nonce, as with any ordinary contract creation
c) The factory's address, a chosen salt and the creation code
d) The factory's address, its nonce and the timestamp of the deploying block

05. NatSpec comments (/// or /** ... */ with tags such as @notice and @param) are written in production contracts mainly to:
a) hide function logic from anyone reading the verified source
b) enforce parameter checks at runtime in place of the require statements developers write by hand
c) tell the compiler which functions to optimize for lower gas
d) document intended behavior for users, maintainers and auditors in a machine-readable form

06. A deployment script submits a contract-creation transaction, receives its transaction hash and prints the address the contract is expected to have. Before publishing that address to users, the team wants proof that a working contract actually exists there.
Which check gives that proof?
a) Read the ether balance at the address and confirm that it is zero
b) Confirm that the node accepted the transaction and returned a transaction hash for the deployment request
c) Confirm a receipt exists for the transaction, whatever its status field reports
d) Fetch the code stored at the address and check it matches the expected runtime bytecode

07. Two nodes for a new private network were initialized from the same genesis file and started with the same network ID on separate machines. Both are running, but neither reports any peers.
What should the operator do?
a) Register each node's enode address with the other as a static peer or bootnode
b) Expose each node's JSON-RPC interface on all network interfaces so the other can find it
c) Enable block production on both nodes, since peers connect only to block producers
d) Give the second node a different network ID or genesis file

08. An upgradeable logic contract replaces its constructor with this function:
function initialize(address _owner) public { owner = _owner; }
The proxy is deployed in one transaction, and the team calls initialize through it in a later one.
What is the risk?
a) initialize runs automatically at deployment, so the team's later call reverts
b) The owner value is written to the logic contract's storage and then ignored by the proxy after every later upgrade
c) Anyone can call initialize first, or call it again later, and make themselves the owner
d) Public functions cannot write state variables in an upgradeable contract

09. Before writing any Solidity, a team writes down properties such as "the sum of all user balances always equals the total supply" and "no withdrawal can pay out more than the caller deposited".
What is the main value of stating these invariants up front?
a) They lower deployment gas, since the compiler can drop redundant checks from the final bytecode
b) They let the EVM reject any breaking transaction without extra code
c) They remove the need for unit tests, because the compiler proves each property before deployment
d) They give tests, audits and assert checks a precise statement of correct behavior

10. A contract pays a reward to the first account that submits the correct answer to a puzzle through submit(string answer). Pending transactions are visible to other network participants before they are included in a block.
Which design best stops an observer copying a correct pending answer and getting it included first?
a) Declare the answer parameter private so the transaction hides it
b) Submit a salted, sender-bound hash first and reveal the answer later
c) Store the submitted answer in a private state variable before comparing it
d) Require a high gas price on each submission so that honest users win first