Past question concernimg offers

In relation to the UNCISG, acceptance is not effective if the indication of assent does NOT reach the offeror:
A) by the same method that the offeror despatched the offer
B)in reasonable time of the offer being made
C)before the time fixed by the offeror
D) before the notice of revocation reaches the offeree

I have no idea if the answer is B or C and i’m very confused. Thanks in advance!