GAUSS7 1998 Problem 25

Two natural numbers, p and q, do not end in zero. The product of any pair, p and q, is a power of 10 (that is, 10,100,1000,10\:000,\dots). If p>q, the last digit of p-q cannot be

Answer Choices
A. 1
B. 3
C. 5
D. 7
E. 9