which of the following could be the greatest common factor of positive integers a and b?
I. 1
II. a
III. a-b  Choices
A) II only
B) I and II only
C) I and III only
D) II and III only
E) I, II and III
It could be 1 if a and b are co-prime, for example, 1 and 2, 11 and 13...   It could be a itself if a is a factor of b, for example, 2 and 4, 5 and 15...   It could be a-b, for example, when a=b+1: a=2 and b=1, a=3 and b=2...   Answer: I, II and III.
<result>E</result>
which of the following could be the greatest common factor of positive integers a and b?
I. 1
II. a
III. a-b