The contrapositive of the answer choice (NOT C - > NOT X) is X - > C
If we have X - > C and connect that to the first premise, then we have C < - X -most- B
Working backwards through a most arrow gives us a some relationship. Because the current relationship between X and B is most Xs are Bs, we know that some Bs are Xs.
C < - X -some- B
Working through this diagram, we can conclude that: C -some- B (equivalent to B -some- C)
These are definitely the most difficult diagrams to work with, but I hope this helps!