Please answer the following question: The immune system is the body's defender. It identifies, tracks down, and destroys troublemakers before they can hurt the body. Those troublemakers may be, for example, bacteria from 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 these invaders before they destroy us. Say you picked up a flu virus last week. Perhaps it entered your body through a cut, from a drinking glass, or from the air you breathe. Of course, you didn't feel it, but the virus made its way into 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 illness. 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 _ 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 the period at the end of this sentence. Your body holds about a trillion --that's 1,000,000,000,000 of them, or about 3,000 in every drop of blood. _ Some of these lymphocytes pass through a small walnut-sized organ called the thymus. The thymus is the base of the neck. Here, special hormones turn lymphocytes into fighting cells, called T cells. T cells have one terrific talent: They can tell the difference between friend and foe; what should be in our bodies and what shouldn't. They do not affect the body 's healthy cells. Yet they attack everything that is foreign to our bodies, such as germs, transplants, and even our own cells which have become abnormal, as in the case of cancer. Which of the following questions best summaries the main idea of the whole passage? A) How does the immune system attack the flu virus? B) How do the lymphocytes work in our body? C) How does the immune system function in our body? D) What is the so-called immune system?
A:
C