Question

We were going out for a walk ________ it began to rain.

A．while

B．before

C．a(chǎn)s

D．when

Answer 1

D

試題分析：考查固定句型。固定句型：Be doing sth when…正要做某事，在這時(shí)（發(fā)生意料之外的事情）。When在…那時(shí)（并列連詞）。句意;我們正要出去散步，在這時(shí)開始下雨了。故D正確。
點(diǎn)評(píng)：when作為并列連詞，意為：在那時(shí)；常與be about to do sth; be doing sth 連用。When后面接意料之外的事情。