1, as I analyzed before, like this case, A and B, C, D, there is no labor relationship, just A and B is a contracting relationship between B and C is an employment relationship, C to find D, because it is the B to the department of the work and subcontracted to the C, so C, D, should also be an employment relationship, so that should be by the C is responsible for the D, and the D should know that changing the window is also a dangerous work, the D should know that the window is also a dangerous work, the D has a major fault in this. Lunch and drink, so D has a major fault in this, he should bear the corresponding responsibility.
2, but in this case, due to the ABCD between the whole did not sign a contract, out of this kind of thing, it is estimated that B will not admit that he subcontracted the work to C, C will not admit that he hired D to help do, so the final estimate will be, C said D is their own to help B to do, the final responsibility by the B and D respectively.
3, if A and B to sign a supplementary contract, that is possible. But from the legal analysis, A has no responsibility in this case.