The computer system suddenly while he was surfing the Internet. A. broke upB. broke outC.

问题补充：

The computer system suddenly while he was surfing the Internet.

A. broke up

B. broke out

C. broke in

D. broke down

答案：

A. broke up

B. broke out

C. broke in

(答案→)D. broke down

解析：短语辨析。A关系破裂；分解；B爆发；C闯入；D机器坏了，身体垮了；分解；抛锚；崩溃；结合句意可知D正确。句意：当他在上网的时候，计算机系统突然就崩溃了。