想問幾個英文小問題 - 學習

可否幫我解釋一下，感激不盡吼~^^


1.A majority of people in the United States can get all the
calcium their bodies require from the food they eat.
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請問為什麼用require？解答上面寫require that .....


2.請問為什麼有時候名詞子句 是"疑問詞+主詞+動詞"
有時候 又變成"疑問詞+動詞...",有辦法可以分辨嗎？ 例如：
a.The behavior of gases is explained by what scientists call the kinetic
theory.

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b.A biologist does not merely descirbe organisms, but tries to learn
what causes them to........
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