旅游经济活动的本质是通过商品交换所形成的（ ）之间的关系。

A．旅游者、旅游接待商、旅游中间商

B．旅游者、旅游接待国、客源输出国

C．旅游者、旅行社、饭店

D．旅游者、旅游经营者、政府

请根据要求改写下列句子，每空一词。

小题1:Her father has been on the phone since half an hour ago. （对划线部分提问）

_________ _________ has her father been on the phone?

小题2:I think diving is the most difficult activity of all. （保持句意基本不变）

I think diving is more difficult than __________ __________ activity.

小题3:We can put those machines on the ground floor of the building. （改为被动语态）

Those machines can __________ __________on the ground floor of the building.

小题4:The beach is a nice place for tourists.  （改为感叹句）

___________ __________ nice place for tourists the beach is!

小题5:Pingping is so energetic that he can be a good monitor. （改为简单句）

Pingping is ___________ ___________ to be a good monitor.