After 1...Kc6 2.d7 the black king will go to b6, which allows the d-pawn to promote (to a queen), winning more material for white after RxQ and RxR. So 2.d7 is a winning move. However if there is a quick way to a forced checkmate, then I don't see it. The solution on the other hand leads to checkmate and is therefore preferable.
ALTERMATIVE (AND QUICKER!) SOLUTION
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