When asked to factor the trinomial x^2 - 18x + 81, a student gives the answer (x - 9)(x + 9). Which of the following statements is true?
A. The answer is incorrect; the trinomial cannot be factored.
B. The answer is incorrect; the plus sign should be a minus sign.
C. The answer is incorrect; the minus sign should be a plus sign.
D. The answer is correct.