They were about to leave it began to rain.
A. before
B. durning
C. while
D. when
网友回答
A. before
B. durning
C. while
(答案→)D. when
解析：考查连词。be about to do sth...when...,正要做某事的时候，这时（突然）...。A.before在...之前，趁...还未，不等...就 B.durning在...期间 C.while 当...的时候，尽管 D.when当...的时候，这时（突然）。句意：他们正要离开时，天开始下雨。