用下列词语造句

需要

    In our city there are more and more cars and buses on the roads. Many people drive to work .First, it's good to have a car. You needn't to wait for the bus. You can go to work on time. Then ,there are many busy roads in the city . Can you drive safely? Traffic lights are very important. You must look at the traffic lights and wait for the green man. If someone is crossing the road, you must stop your car and look carefully. We must follow the rules and stay safe on the road.

下列说法正确的是（   ）

最小的血细胞是____，它没有____，形状____。它主要有____的作用。