题目内容
（请给出正确答案）
[主观题]
Among the many open research questions on MTBE are how it is metabolized, ______the levels
of exposure in the general population are.
A．but what
B．and what
C．but how
D．and why
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A．血水样便
B．脓血便
C．米泔水样便
D．洗肉水样便
E．黄水样或清水样便
