A.after which
B.with which
C.for which
D.at which
A.in which
B.for which
C.with which
D.of whom
A.in which
B.in what
C.of what
D.of which
Their success was due to () they had been working hard.
A.the fact of which
B.the fact
C.the fact which
D.the fact that
A.who
B.whose
C.of whom
D.which