Math problem I can't figure out

x is a positive integer.

Which is larger?

A.The number of multiples of 3 between 100 and x+100.
B.The number of multiples of 7 between 100 and x+100.
C.They are equal.
D.There is not enough information to solve the problem.

I picked A, because I thought no matter what x, 3 will always have more multiples.
However, the answer is D.

Can anyone who gets this explain why D is the correct answer?

Thanks to everyone who answers.

Well - I agree with you

but (why is there always a BUTT?)

if x = -100

THEN C becomes correct

therefore (love these big math wordz)
D has got to be it

all the best