题目

—Did you catch the school bus this morning?—No.It when I arrived at the bus stop.A. is leaving B. has left C. had left D. would leave 答案：【答案】C【解析】句意：——你今天早上赶上校车了吗？——没有，我到达车站时，车已经走了。A. is leaving现在进行时；B. has left现在完成时；C. had left过去完成时；D. would leave过去将来时。leave发生在arrived之前，表示过去的过去，因此用过去完成时。答案为C。(2005年长沙)元素之间最本质的区别是 [　　] A．质子数不同 B．中子数不同 C．核外电子数不同 D．中子数与核外电子数之和不同