One day last November, Liz Douglas went shopping. She spent hours at the supermarket. After she paid for all the things she bought, she went back to her car and put the shopping bags on the back seat (后座) of the car. Then she started to drive home.

    Ten minutes later, it began to rain heavily. Soon, Liz found it hard to see the road clearly. There was a lot of water on the road, so she drove slowly. Although Liz was an experienced driver, she felt afraid.

    突然，Liz看见她前面的一棵树并试图停车。But it was too late. Liz closed her eyes. She hoped that the airbags (气囊) in the car would work when the car hit the tree. Unluckily, they didn't.

    However, Liz was lucky. When the car hit the tree, a bag of bread flew out of one of the shopping bags. The car turned over, and the bread landed between Liz' s head and the roof(顶部) of the car. It stopped her head from hitting the roof of the car.

    About half an hour later, firefighters (消防队员) helped Liz get out of the car. They sent her to the hospital, but she went home soon because she didn't get hurt badly. Her head was free and she only had a few cuts. Liz's son went to get the things Liz bought from the car and gave the bag of bread to her. She was going to keep it as a souvenir (纪念品) because it brought her good luck.

绿眼虫是一种长梭形、绿色的动物，可用来监测水污染，它属于单细胞动物．单细胞动物的特征是（　　）