The world's supplies of copper...a,have been gradually being exhaustedb,have gradually exhaustedc,are gradually exhaustedd,are being gradually exhausted

问题描述:

The world's supplies of copper...
a,have been gradually being exhausted
b,have gradually exhausted
c,are gradually exhausted
d,are being gradually exhausted

a.been和being重复
d.are和being重复
exhausted是一个持续的动作,固应用现在完成时,选B