M₂O₇^x-离子和S^2-在酸性溶液中发生反应：M₂O₇^x-+3S^2-+14H⁺=2M³⁺+3S↓+7H₂O，则M₂O₇^x-离子中M的化合价是（　　）

A．+2

B．+3

C．+4

D．+6

The demand is the good use we ______ our spare time for our study of science and technology. [    ]

A. make from  

B. make of  

C. make out  

D. make into