()Yesterday morning I was just about to leave ___ to telephone rang.

A
when和while 还可作并列连词。when表示“在那时”；while表示“而，却”，表对照关系。如：

a. The children were running to move the bag of rice when they heard the sound of a motor bike.

孩子们正要跑过去搬开那袋米，这时他们听到了摩托车的声音。

b. He is strong while his brother is weak.

他长得很结实，而他弟弟却很瘦弱