If a and b are positive integers and x = 4^a and y = 9^b, which of the following is a possible units digit of xy?  Choices:
A) 1 B) 4 C) 5 D) 7 E) 8
unit digit of x=4^a will either be 4 or 6 (4 if a is odd and 6 if a is even)   and unit digit of y=9^b will either be 1 or 9 (9 if b is odd and 1 if b is even)   now we have total 4 possible combinations   1) a=odd, b=odd   unit digit=9*4=6   2)a=odd, b=even   unit digit=4*1= 
<gadget id="calculator">4*1</gadget>
<output>4</output>
4   3) a=even,b=odd   unit digit=6*9=4   4)a=even, b=even   unit digit=6*1= 
<gadget id="calculator">6*1</gadget>
<output>6</output>
6   thus, only two values are possible for the unit digits (4 or 6). out of these values only 4 is mentioned in the given choices. hence answer must be 4.
<result>B</result>
If a and b are positive integers and x = 4^a and y = 9^b, which of the following is a possible units digit of xy?