题目

——Have you ever         an e - mail         your parents? ——No, they can't use a computer. A. send, to             B. sent, to             C. send, for             D. sent, for 答案：B（经典题）如图所示，锐角△ABC中，BE，CF是高，点M，N分别为BC，EF中点．求证：MN⊥EF．