Internet.

A.broke down B.broke out

C.broke up D.broke in

提示：break down"坏了，出故障，出毛病"；break out"（战争，火灾等）突然爆发"；break up"拆散；打碎；分解；"break in"间入；插话；打断"。而句意为："他正在网上查信息时电脑系统突然坏了。"

答案：A

5. When________ different cultures,we often pay attention only to the differences without noticing the many similarities.

提示：when引导的部分实际上是"when we are comparing different cultures"的省略。当从句主语与主句主语一致时，从句中主语和be动词一起省略。

答案：C