阅读下面短文，从各题所给的A、B、C、D四个选项中选出最佳选项。
A
Characters: Evan, Mrs Bordoni, Mr Contei, Rita, Marco, Tom
Setting: An elementary school
Mrs Bordoni: (Stands at the school gate) Good morning! You need to go straight to your classrooms, children. (A shy boy walks up, looking nervous) What is your name?
Evan: (Looking at the ground) My name is Evan.
Mrs Bordoni: Welcome to Northside. I'm the principal (校长), Mrs Bordoni.
Evan: (Looks surprised) Oh! Good morning.
Mrs Bordoni: I know you are new to our school, but you will like it here. I will take you to your classroom.
Evan: I didn't want to move and leave my friends.
Mrs Bordoni: You will have friends here. You'll see. Here's your room, 106.
Mr Cortez: Welcome to our class! I'm Mr Cortez.
Evan: (The class looks at him. He speaks softly.) Hello.
Mr Cortez: We are talking about a story we just read. Who can tell Evan what it was about?
Rita: This family moves to a new state. Their story is both funny and sad.
Evan: (Cheering up) Did they like the new place?
Marco: Not at first. But they made lots of new friends.
Mr Cortez: What happened to make them change their minds?
Tom: The girl from next door said he needed help. When they got to her house, a sign said, "Welcome".
Marco: It was a surprise party for them. The whole family was laughing.
Mr Cortez: The neighbors made the family feel welcome.
Mrs Bordoni: (Stands in the doorway) Evan, it's time for lunch. (They enter the lunchroom. A sign says, "Welcome Evan".) It is hard to change schools and leave old friends. Now, we are your friends. Evan: (Smiles) I can't believe you did this for me.
P）和α粒子（
H
）以相同的速度垂直进入同一匀强磁场中，它们在垂直于磁场的平面内做匀速圆周运动，它们的轨道半径和运动周期关系是 ( )
