Ethereum Smart Contracts BBLC Practice Questions (Page 3)

Questions 11–15

1. 11application · medium

   A developer has written a Solidity smart contract that manages a token sale. The contract includes a constructor that sets the owner, a state variable for the token price, and a function that allows users to purchase tokens by sending Ether. The developer wants to deploy this contract to the Ethereum mainnet. What is the primary action the developer must take to make the contract available for users to interact with?

   • ASend Ether to the contract's constructor address to trigger automatic deployment.
   • BCompile the contract and submit a transaction containing the bytecode to the Ethereum network.
   • CPublish the Solidity source code on a public repository so users can compile it locally.
   • DCreate a new block on the Ethereum network that contains the contract's ABI.
   Reveal Solution Discussion Select an answer first
2. 12application · medium

   A supply chain dApp stores a large array of tracking data on-chain. Users report that transactions calling the function to update a shipment status are frequently failing due to 'out of gas'. The contract's state variables are updated inside a loop that iterates over the entire array. What is the most effective way to reduce gas consumption for this function?

   • AIncrease the gas limit on the user's wallet settings.
   • BStore the tracking data off-chain and only keep a cryptographic hash on-chain.
   • CUse a more recent version of the Solidity compiler.
   • DRemove the loop and update only the last element of the array.
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4. 13application · medium

   A developer is writing a Solidity contract for a crowdfunding campaign. The contract needs to store the funding goal, track contributions, and allow the owner to withdraw funds once the goal is met. Which set of components is essential?

   • AState variables for the goal and contributions, a function to contribute, and a function to withdraw.
   • BA state variable for the owner, a function to contribute, and an event for withdrawals.
   • CA library for safe math, a constructor, and a selfdestruct function.
   • DAn event for each contribution, a modifier to restrict access, and a fallback function.
   Reveal Solution Discussion Select an answer first
5. 14foundation · easy

   What is 'gas' in the context of Ethereum smart contracts?

   • AA fee that is paid to the contract developer for deploying the contract.
   • BA type of token that is used to vote on network upgrades.
   • CA unit that measures the computational effort required to execute operations on the Ethereum network.
   • DA storage space on the blockchain that is used to save contract data.
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6. 15expert · hard

   A smart contract for a decentralized exchange has a function that allows users to swap tokens. The function uses the `transferFrom` method of an ERC-20 token. An attacker is able to drain tokens from the contract by repeatedly calling the swap function before the first call completes. What is the most likely vulnerability?

   • AReentrancy, because the contract does not update its state before making external calls.
   • BInteger overflow, because the token amount is stored in a uint8.
   • CGas exhaustion, because the swap function consumes too much gas.
   • DAccess control, because the swap function is not restricted to certain users.
   Reveal Solution Discussion Select an answer first