通读下面短文，从题中所给的 A、B、C三个选项中，选出最佳选项填空，使短文意思通顺、结构完整。

What makes us ill? It is the germs（细菌）. 1are everywhere. The are very 2. You can't find them with your eyes, but you can see them 3 a microscope（显微镜）. There could be hundreds of them on a very small thing.

Germs are always found in 4water. When we look at dirty water under the microscope, we shall see 5. So your father and mother will not let you drink dirty water.

Germs are found not only in 6. They are found 7 in air and dust（灰尘）. When you 8 your finger, if some dust from the floor goes into the cut, some germs would go into your finger. Then you will have much pain in it. Sometimes germs would go into all of your body, and you would have pain everywhere.