读短文，写出正确的国家。

      I'm dreaming to visit the U. S., Canada, the U. K. and Australia. My plane is flying to the west from Beijing.

So the first country I will visit is ____________. Then my plane is still flying to the west. I can see beautiful

Rocky Mountains on the plane. Now I arrive in ____________. Then I arrive in ____________, the president

Obama meets me at the airport. At last, I land on ____________'s land. Many kangaroos are around me. I'm

glad to see them.

明明从广州给远在上海的爷爷打电话，电话费随着时间的变化而变化，在这个过程中，因变量是[ ]

A．明明

B．电话费

C．时间

D．爷爷