阅读短文，从下面所给的选项中选出可以填入空白处的最佳选项，其中有一个多余的选项。

That one looks like a rabbit. Or is the larger one a whale? Whatever shape they take, they’re clouds. So what are clouds, exactly? ____1____

Clouds are floating (浮动的) water.

Clouds are made of small water drops. Each drop is made of many flying water molecules (分子). They stay together around a tiny piece of dust (尘埃) or salt. How small are cloud drops? A shoebox of clouds might hold millions of cloud drops.

____2____

Clouds look light and soft, but they hold much water. And water is heavy. The water in an average cotton-ball cloud may weigh as much as a group of elephants.

Things live in clouds.

Have you ever dreamed of living on a cloud? It would probably be cold and wet, with no place to sit down. But weather balloons have discovered insects inside clouds as high as 4 miles above the earth. ____3____ Airplane pilots have ever found butterflies at 10,000 feet.

Clouds don’t last.

Clouds are always changing. They form and then change into gas soon. They are changing shapes as the wind blows them across the sky. Will the shapes stay long? ____4____ But some small soft clouds often last 10 minutes.

So take a look at the cloud before it disappears—what does that cloud look like?

A. Clouds are heavy.

B. Clouds look like elephants.

C. Some big clouds can hang around all day.

D. The following can help you know more about them.

E. Most of the insects inside clouds are blown there by the wind.