In the past decades sea ice in the Arctic as a result of global warming.A. had meltedB.

问题补充：

In the past decades, sea ice in the Arctic as a result of global warming.

A. had melted

B. has been melting

C. melted

D. is melting

答案：

A. had melted

(答案→)B. has been melting

C. melted

D. is melting

解析：考察时态。本题的关键是前面的时间状语In the past decades。只要出现“in the past/last+一段时间”，都要和现在完成时连用。句意：在过去的几十年里，由于全球变暖，北极的海冰一直都在融化。B项的现在完成进行时表示从过去开始一直延续到现在的情况。这一情况符合题意。