Ethereum Virtual Machine BDC Practice Questions (Page 8)

Questions 36–40

1. 36application · medium

   A developer is designing a contract that needs to generate random numbers. They plan to use the block timestamp and the contract's address as sources of entropy. What is the main risk of this approach?

   • AThe EVM does not support arithmetic operations on timestamps.
   • BThe block timestamp is not available during execution.
   • CMiners can manipulate the timestamp to influence the outcome.
   • DThe contract address is not deterministic.
   Reveal Solution Discussion Select an answer first
2. 37application · medium

   A developer is comparing two implementations of a function. Implementation A uses a loop to sum an array of 100 elements. Implementation B uses a single arithmetic expression that computes the sum in constant time. Which implementation is likely to have lower gas costs?

   • ABoth have the same gas cost because they perform the same calculation.
   • BImplementation A, because storage access is cheaper than arithmetic.
   • CImplementation A, because loops are optimized by the EVM.
   • DImplementation B, because it uses fewer opcodes.
   Reveal Solution Discussion Select an answer first
4. 38expert · hard

   A smart contract function is designed to update a user's balance and then call an external contract. The external contract's function is known to sometimes revert. The developer wants to ensure that if the external call reverts, the user's balance is not updated. Which pattern should be used?

   • AUpdate the balance first, then make the external call.
   • BMake the external call first, then update the balance.
   • CUse a try/catch block to handle the external call revert and only update the balance if the call succeeds.
   • DUse a low-level call and ignore the return value.
   Reveal Solution Discussion Select an answer first
5. 39foundation · easy

   Which opcode is used to add two 256-bit integers from the stack?

   • ACALL
   • BMUL
   • CADD
   • DPUSH
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6. 40foundation · easy

   What is a direct consequence of the EVM being isolated from the outside world?

   • ASmart contracts cannot store any data on the blockchain
   • BSmart contracts are executed on a single node only
   • CSmart contracts can only be written in Solidity
   • DSmart contracts cannot directly access external APIs or network resources
   Reveal Solution Discussion Select an answer first