There are only two ways to tell somebody thanks: Kudos and Marked Solutions Unofficial Forum Rules and Guidelines "Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God" - 2 Corinthians 3:5
A is the Answer. Besides many other possible explanations one explanation could be: In B, C and D we change Input A and haven’t the original value for input A anymore. But in the equation we need the original value for A so B, C and D can't be the correct answer.