T-4-Q33 一点小疑问。请大家帮忙

In attempting to solve the problems caused by a lowering of the price of oil, oil companies operating in the North Sea have taken a variety of approaches, which includes their reducing employment, using new technology to pump oil more efficiently from smaller fields, and finding innovative way to cut the cost of building and operating platforms.
            

B.      which includes reducing employment, using new technology to be more efficient in pumping oil

C.      which include reducing employment, using new technology to pump oil more efficiently

D.      which include the reduction of employment, their using new technology to pump oil more efficient

E.       including a reduction of employment, their use of new technology to be more efficient at pumping oil

答案是c选项。  这里include为复数形式是因为 which 修饰approaches， 还是因为修饰的是a variety of， 但 a variety of +复数， 动词用复数啊