The little girl can’t decide ______ though there are so many kinds of MP5 in the mall.

A．what to choose

B．to choose which

C．which to choose

D．to choose what

气孔是植物体与外界进行气体交换的“窗口”．气孔的开闭受（　　）控制．

A．表皮细胞

B．叶肉细胞

C．保卫细胞

D．保护细胞