Can "which" modify a clause?

hey,
This question is from the Gmatclub grammar book,

36. The house plants that mom set out on the patio yesterday are dead which determines we must had had frost last night.
a) which determines we must had had frost
b) which means it must have frosted
c) that goes to show the frost must have been
d) and this proves the frost has been falling
e) showing that it had been frosting

The correct answer is b,
I don't understand how "which means" modify the precceding clasue? is it ok grammatically?
thanks :D