When asked to factor the trinomial x2 - 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 minus sign should be a plus sign.
c. the answer is incorrect; the plus sign should be a minus sign.
d. the answer is correct.