_____is it that is troubling Mary ?—-Working out the maths problem..

_he lives in a large flat and owns much money, he doesn’tseem as happy as before．

I would like to buy a present ____ my cousin and I must give it ______her myself.

Thanks to the application of new farming methods,output is now six times _____it was before 2000.

The earth（地球） is our home, and we must _____ it clean.