研究对象包含病例和对照两部分，但两部分的信息均来自于同一个个体（） A．巢式病例对照

     Many people catch a cold in the springtime or fall. It makes us wonder… if scientists can send a man to
the moon, why can't they find a cure for the common cold? The answer is easy. There are actually
hundreds of kinds of cold viruses out there. You never know which one you will get, so there isn't a cure
for each one.
     When a virus attacks your body, your body works hard to get rid of it. Blood rushes to your nose and
cause a block in it. You feel terrible because you can't breathe well, but your body is actually eating the
virus. Your temperature goes up and you get a fever, but the heat of your body is killing the virus. You also have a runny nose to stop the virus from getting into your cells. You may feel miserable, but actually your
wonderful body is doing every- thing it can to kill the cold.
     Different people have different cures for Colds. In the United States and some other countries, for
example, people might eat chicken soup to make themselves" feel better. Some people take hot baths and
drink warm liquids (流汁). Other people take medicine to stop Various symptoms (症状) of colds.
     There is one interesting thing to note" some scientists say taking medicines when you have a cold is
actually bad for you. The virus stays in you longer because your body doesn't have a way to fight it and kill
it.
Bodies can do an amazing job on their own. There is a joke, however, on taking medicine when you have a cold. It goes like this:
     It takes about one week to get over a cold if you don't take medicine, but it takes only seven days
to ~et over a cold if you take medicine.