补全对话。

A: Excuse me, is that your key?

B: 1_____, it isn't. It's Bill's.

A: OK. Bill, is this your 2_____ ?

C: Let 3_____  see. No, it 4_____. It is Bob's.

A: But 5_____  Bob?

C: He's 6_____  home now. You can 7_____ him at 839-0742.

A: 8_____  you.

C: You are welcome.

在人体的血液循环中，对运载血细胞和运输养料和废物起重要作用的是（ ）

A．红细胞

B．白细胞

C．血小板

D．血浆