What happens when sea water touches different elements another subject of study.

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问题 What happens when sea water touches different elements      another subject of study.

选项 A、are
B、is  
C、have been
D、was

答案B

解析 本题首先要搞清楚句子的结构。What happens是主语;when sea water touches different elements是时间状语;another subject of study是表语。因此空格处缺少系动词。What happens是名词性从句作主语,谓语动词通常用单数形式。另外,本题的主语从句和时间状语从句都用的一般现在时态,依据前后一致原则,空格处也应该是一般现在时。所以答案选B
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