（10南京市3）·金属钒被誉为“合金的维生素”。五氧化二钒(V₂O₅)中钒元素的化合价为

A．一3

B．+ 1

C．+ 3

D．+5

To ____   this happening again, John commanded that the source of all the water supplies    ___  .  [      ]

A. stop, was examined                  

B. keep, should be examined  

C. prevent, examined                  

D. stop, be examined