题目

I felt it hard to keep up with my classmates. But whenever I want to ____________, my teacher always encourages me to work harder.        A. go on                B. give up                     C. run away                  D. give back 答案：B力F1和F2是同一直线上的两个力，它们合力大小是30N，已知F1为20N，那么F2的大小可能是________．