阅读短文，选出合适的一项。

A: picture

A: Look! I have a picture. What can you see on the wall, Tim?

B: I can see a map of China.

A: Where is Wuhan? Do you know?

B: Sorry, I can't find it.

A: Where's Beijing?

B: It's here.

A: Can you see the children?

B: Yes, I can. Who are they?

A: Look, the two American girls are Lucy and Lily. The flowers are in Lucy's hands, Lily is behind the desk. The girl in red is Han Mei. The boy is Wang Ming. His kite is on the wall. That's Jim. His football is under the desk.

小汽车的外形设计成流线型（如图所示），当小汽车在水平路面上高速行驶时对路面的压力比静止时____ （选填“增大”、“减小”或“不变”），其原因是____ ．