在生物实验操作中，小海在观察人血涂片时，若要在视野中观察到较多的血细胞，应该选择显微镜的放大倍数是（  ）

A:____?

B: I'm reading a book.

A:____?

B: It's about a great player. Her name is Deng Yaping.

A: Is she your hero?

B____. I admire her very much.

A:____?

B:She's won many world competitions, including four gold medals in the Olympic.

A:What did she do after that?

B:She began to study at Tsinghua University in Beijing and then attend university abroad.

Whatever she does ,she never gives up.

A:____

 某工人用如图所示的装置把一重为1200N的箱子从斜面底端匀速拉到顶端用时10秒，已知斜面长6m、高2m，此装置的机械效率为80%（滑轮重、绳重、滑轮与绳之间的摩擦均不计）．求：