The answer is "A" because whenever you round (up or down) you are only changing the value a teeny bit, which only matters if you want to measure an exact amount of something. Using a calculator gives you the exact answer. Guess and check really doesn't work because there are ∞ numbers. Any answer is not fine because if the answer is 5 and you answer with 9633, that answer is obviously wrong. So, A is right.