---- she came in,i was writing a letter A when B while C sin

A
题意：她进来时,我正在写信.
再问： ____ there is no rain,it's difficult to plant trees A where B when C which D How 到底选 A,还是B？？ 就这一题了！！好了就采纳！~\(≧▽≦)/~啦啦啦 今天晚上做好！
再答： B when在此引导时间状语从句，但有条件的意义。 题意：如果没有雨水，植树不有点困难了。