用括号内所给单词的适当形式填空。

    1. My pen pal wants me____________(buy)a ticket to the 2012 London Olympic Games.

    2.1 need to get some money__________(pay) for the new bike.

    3. Don't let the students__________(do) lots of homework.

    4. I don't want to___________(surprise) Lisa.

    5. Parents should give their children more time and___________( free) to relax.

Ficoll密度梯度分层液主要用于分离外周血中的

A．粒细胞
B．单个核细胞
C．红细胞
D．巨噬细胞
E．血小板