问题补充：
We were swimming in the lake suddenly the storm started.
A. when
B. until
C. while
D. before
答案：
(答案→)A. when
B. until
C. while
D. before
解析：考查固定句式。固定句式：be about to do sth/be doing sth when…正要做某事/正在做某事 在这时….；when后面引导的是一个并列句，when指“在这时”；句意：我们正在湖里游泳，在这时突然下起了暴雨。故A正确。
