题目内容：
根据以下资料，回答题。
Many people catch a cold in the springtime and/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 literally 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 brings congestion with it．You feel terrible because you can't breathe well, but your body is actually "eating" the virus．Your temperature rises 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 to your cells．You may feel miserable, but actually your wonderful body is doing everything it can to kill the cold.
Different people have different remedies for colds．In the United States and some other countries, for example, people might eat chicken soup to feel better．Some people take hot baths and drink warm liquids．Other people take medicines to stop the fever, congestion, and runny nose．
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:
If takes about one week to get over a cold if you don't take medicine, but only seven days to get over a cold if you take medicine.
The reason that scientists cannot cure a common cold is __. A． we don't know how we get cold
B． we aren't sure the exact kind of cold viruses
C． we don't know when we will catch cold
D． we aren't sure the exact scientific method
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