After the discussion,I forced myself to trust the answer is D,since the tricky explaination conviced me generally.In this case,if "are" is added into the structre ,the meaning will be more clear as YAOYAO said above.C is not a wrong structure but a worse one when compared with choice D.
After the discussion,I forced myself to trust the answer is D,since the tricky explaination conviced me generally.In this case,if "are" is added into the structre ,the meaning will be more clear as YAOYAO said above.C is not a wrong structure but a worse one when compared with choice D.