题目内容：
根据以下内容，回答题。The immune system is the body's defender. It identifies, tracks down, and destroystroublemakers before they can hurt the body. Those troublemakers maybe, for example, bacteriafrom a cut or splinter, a measles germ, a cold bug, or even a cancer cell.
These invaders try to take over our tissues and feed off our bodies' nutrients. If they succeed,we become sick or, sometimes, even die. It's the job of our immune systems to destroy theseinvaders before they destroy us.
Say you picked up a flu virus last week. Perhaps it entered your body through a cut, from adrinking glass, or from the air you breathe. Of course, you didn't feel it, but the virus made its wayinto your bloodstream. As soon as it entered your body, it began to reproduce. Viruses have only one goal: to take over your cells. Once inside your body, viruses try to enter cells and disrupt their normal work. If left alone, these viruses would hurt so many cells that you would weaken, or worse,get a serious disease.
But this flu virus should not be so complacent as it seems to be. As it reproduces in your bloodstream, the virus is met by a certain kind of white blood cell, the lymphocytes. The lymphocytes are the foot soldiers that keep you alive.
The number of lymphocytes in your body is hard to imagine. Thousands of them could fit in theperiod at the end of this sentence. Your body holds about a trillion--that's 1000000000000 ofthem, or about 3000 in every drop of blood. Since you began reading this sentence, over 800000 ofthem have been created and destroyed.
Some of these lymphocytes pass through a small walnut-sized organ called the thymus. Thethymus is the base of the neck. Here, special hormones turn lymphocytes into fighting cells, calledT cells.
Tcells ove one terrific talent:they can tell the difference between friend and foe ;what shouldbe in our bodies and what shouldn't. They do not affect the body's healthy cells. Yet they attackeverything that is foreign to our bodies, such germs, transplants, and even our own ceils which havebecome abnormal, as in the case of cancer.
What is the function of our immune system? A．To take over our tissues and feed off our bodies'nutrients
B．To prevent bacteria or germs from entering the body
C．To destroy the troublemakers in order that they might not hurt the body
D．To fight against T cells
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