购买力平价理论认为，汇率的变动是由( )变化引起的。

A．两国购买力之比

B．两国购买力之积

C．两国购买力之差

D．两国购买力之和

I don't want to buy this pair of black shoes. I want to buy ________.         [ ]

A. that one

B. that pair

C. that ones

D. that a pair