Among the many open research questions on MTBE are how it is metabolized, ______the levels of exposure in the general population

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问题 Among the many open research questions on MTBE are how it is metabolized, ______the levels of exposure in the general population are.

选项 A、but what
B、and what
C、but how
D、and why

答案B

解析 需要 what 从句结构。
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