An empty set is a subset of any set, so choose C.

the second question

Presumptive sufficiency:

Pass 0

Re-push the necessity:

Pass a< 1/b, then a may be 0, b >；; 0, then 0

So choose d

Besides, the answer on the first floor is wrong. An empty set is a subset of any set, and it is the proper subset of a non-empty set!