medicine works in a human body is a question not everyone can understand fully.
A. How; that
B. That; which
C. That; which
D. What; that
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(答案→)A. How; that
B. That; which
C. That; which
D. What; that
解析：考查名词性从句和定语从句。从句How medicine works in a human body在句中做主语。定语从句That not everyone can understand fully修饰先行词question，that指代question在句中做understand的宾语。句意：药物如何再人体中起作用是一个不是所有人都能完全理解的问题。