—Have you thought of a topic for your speech yet? —No, I______a lot about it, but I haven’t decided. A. will think B. have been thinking C. had thought D. was thinking 答案:B中国某集团军进行模拟抗震救灾演练,现在一批救灾物资需运往距部队驻地120公里的灾区,需要1小时走了50公里后,连长发现在以此速度不能按时到达,于是部队加速前进,请问后半小时速度至少为多大才能保证按时到达?