阅读理解。
It is Saturday afternoon. Liu Tao and his friend David are taking a walk in the park. There are many
public signs in it. They mean different things. Liu Tao knows a lot of things about them. Suddenly, they
see a short man walking on the grass. Liu Tao points to the sign on the grass and says to him, "Sir, look
at the sign! You can't walk on the grass. "The man smiles and says, "You are right, young man. But I'm
the park keeper. I'm looking after the grass."
根据短文内容，判断下列句子正(T)误(F)。
( )1. It is Saturday afternoon.
( )2. The boys can see many public signs in the park.
( )3. Liu Tao doesn't know the things about the public signs.
( )4. The man can't see the public signs in the park.
( )5. The man is the park keeper. He wants to take a walk on the grass.
匀速下落，并进入下边区域（该区域的电场足够广），在如图所示的速度--时间图象中，符合粒子在电场内运动情况的是
