If a and b are positive integers and x = 9^a and y = 5^b, which of the following is a possible units digit of xy?  Choose the correct choice from the following answers.
A) 1  B) 4  C) 5  D) 7  E) 8
The units digit of 9^a is either 1 or 9.   The units digit of 5^b is 5.   1*5= 
<gadget id="calculator">1*5</gadget>
<output>5</output>
5 and   9*5= 
<gadget id="calculator">  9*5</gadget>
<output>45</output>
45.   The units digit of xy is 5.   The answer is 5.
<result>C</result>
If a and b are positive integers and x = 9^a and y = 5^b, which of the following is a possible units digit of xy?