Our Muscles
1．When we think of muscles, we might often picture a bodybuilder with big arm and leg muscles. But our muscles do not have to look like that to work well.
2．As a matter of fact, every time we write our name, we use the 20 different muscles in our hand. Every time our heart beats, we use our cardiac muscle（心肌）When we eat our food, we use our tongue muscles. We even use muscles when we open and close our eyes. In fact, every time we move, we use some of the muscles in our body.
3．A muscle is made of tiny fibers. Fiber is a type of tissue that feels a little like a rubber band. Thousands of these fibers are packed together to make a single muscle. We all have the same number of muscles—about 700 or so. Men and women with big muscles simply have thicker bundles of fibers.
4．There are three different types of muscles in our body. Smooth muscles are the ones that we cannot control. They work quietly to keep our body running. Also, they are the ones that help us to absorb our food. The cardiac muscle makes the heart pump blood in and out. Skeletal muscles are the ones that we can control. These are the muscles that we use to raise our hands, or ride our bicycles.
5．Even our face often uses muscles. But if we want to save our energy, try smiling instead of frowning. Remember that it takes 17 muscles in our face to smile, but 43 muscles to frown!
Paragraph 5______.
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
【正确答案】：F
【题目解析】：第五段第二句是本段的中心句，讲的是微笑比皱眉要节省能量，因为皱眉时所用到的肌肉数量更多，可知我们要多微笑少皱眉。选项F与本段对应。
