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!
Our muscles are working______.
A、frowning
B、a lot of fibers
C、to digest food
D、to function properly
E、whenever we are happy
F、whenever we do something
【正确答案】：F
【题目解析】：题干的中心词是"working"。第二段最后一句说只要我们活动，我们就会用到某些肌肉。本题是说“不管我们在做什么，我们的肌肉都在工作”。选项F符合题意。
