Factor.3a^2(a-b)-6a(a-b)+21(a-b)

Please assist me with the proper way to do this problem.

Possible answers: A: (3a – b)(a^2 – 2a + 7), B:3(a – b)(a^2 – 6a + 21),
C:(a – b)(3a^2 – 6a + 21)D:3(a – b)(a^2 – 2a + 7).

Thanks