Children’s brains can’t develop properly_______ they lack protein(蛋白質).
A．whenB．sinceC．becauseD．unless
A
 本題考查when的一個特殊用法。在本題中的when引導的是一個條件狀語從句，相當于if。整個句子的意思是：如果孩子缺少蛋白質，大腦就不能正常發(fā)育。
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