I was about to inform you of the sudden change an unexpected visitor came.A. becauseB. bef

问题补充：

I was about to inform you of the sudden change an unexpected visitor came.

A. because

B. before

C. when

D. unless

答案：

A. because

B. before

(答案→)C. when

D. unless

解析：固定句式：be about to do sth when…正要做某事在这时…；本句中的when是一个并列连词，引导的是一个并列句，表示意料之外的发生的事情。句义：我正要通知你那一个突然的变化，在这时一名不速之客突然来访。故C正确。