For each polynomial operation, write the letter that corresponds to the resulting expression from the list. (5x2 + 2x + 1) + (6x2 − 3x + 9) is equivalent to expression . (-3x2 + 6x − 12) + (5x + 9) is equivalent to expression . (8x + 16) − (3x2 − 3x − 15) is equivalent to expression . (5x2 + 23x − 7) − (x2 − 4x + 21) is equivalent to expression