计算：

（1）a^m-1÷a^m-2；

（2）8a⁶b⁵÷（-2ab）³；

（3）（6x³y²-7x⁴y）÷xy．

Over the past decades, sea ice _________ in the Arctic as a result of global warming.  [      ]

A. had decreased        

B. decreased        

C. has been decreasing  

D. is decreasing