If a and b are positive integers and x = 7^a and y = 3^b, which of the following is a possible units digit of xy?	Choose the correct choice from the following answers.
A) 2
B) 5
C) 6
D) 8
E) 9
The units digit of 7^a is either 7, 9, 3 or 1.   The units digit of 3^b is 3, 9, 7, or 1.   We can multiply the units digits together to see that the units digits of xy could be 1, 3, 7, or 9.   The answer is 9.
<result>E</result>
If a and b are positive integers and x = 7^a and y = 3^b, which of the following is a possible units digit of xy?