补全下列对话：

A: Hi! Sonia. This is your letter.

B: 1.________ Oh, it's 2.________  my pen pal, Mike.

A: What's in it?

B: There are some 3.________  in it. Look! In the first photo, he's 4.________  basketball at school. In the second photo, he's swimming at the 5.________ .

A: How nice! What are they 6.________  in the next photo?

B: They're 7.________  dinner.

A: And the last photo?

B: He's 8.________  his sister, Gina. Gina is doing 9.________  homework. Mike is 10.________

说明样本决定系数的含义及其作用。