问题1选项
A.What a muscle consists of
B.When we use our muscles
C.How people think of muscles
D.What kind of muscles we have
E.Why we have different muscles
F.Why we should smile rather than frown

D

第四段第一句是本段的中心句，讲的是我们体内有三种不同类型的肌肉。接着又分别讲每种肌肉的具体功能。选项D与本段对应。