If A = {1, 2} and B = {1, 2, 3}, then A - B is:
Answer: B
The set difference A - B contains all elements that are in set A but not in set B.
A = {1, 2}
B = {1, 2, 3}
All elements of A (1 and 2) are also present in B. Therefore, there are no elements that are in A but not in B.
A - B = ∅ (the empty set).