Olofsson’ s corpus analysis finds that the following semantic environments favor certain relative pronouns
Relativized nonhuman subjects: that
Relativized nonhuman objects : no relative pronoun (it’s normally deleted)
Relativized prepositional objects : which
(Note that the final environment reflects a grammatical rule observed in written English rather than a usage tendency or preference, since prepositions are generally fronted along with relativized prepositional objects in writing and since only which can occur after prepositions. )