B is not a drawback, so it can't be that.
C is not a bad answer, both could be inefficient in terms of cost. And a windmill does take maintenance.
D is not a horrible answer, considering some homes may be in places that don't get enough sun and/or wind. Like if you lived in the boreal forest in Canada, or on the forest floor in the Amazon jungle.
References :
If enough households demand an individual windmill and solar panel, the initial costs will come down to reasonable expense!!
References :
Barack Obama's Smart Grid Plan.