The terms of an arithmetic progression are written in reverse order before adding them. If the original progression has first term 12, common difference 3, and 15 terms, which statement about the sum is correct?
(a) The sum becomes half of the original sum.
(b) The sum becomes double the original sum.
(c) The sum remains the same as before reversing the order.
(d) The sum changes because the common difference turns into −3, so the sum decreases.