If a and b are positive integers and x = 9^a and y = 3^b, which of the following is a possible units digit of xy?  Choose the correct choice from the following
A) 2  B) 4  C) 5  D) 7  E) 8
The units digit of 9^a is either 1 or 9.   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 7.
<result>D</result>
If a and b are positive integers and x = 9^a and y = 3^b, which of the following is a possible units digit of xy?