题目

It will be a long time ____Frank _______back from abroad.   A. before; comes         B. since; has come      C. when; has come        D. after; came 答案：A20．甲区域的居民可以沿图示箭头到达附近城市寻求服务，通常情况 下，到达频率最高和最低的城市分别是（　　）A．u 和 sB．u 和 tC．v 和 sD．v 和 t