If a is a positive integer, and if the units digit of a^2 is 1 and the units digit of (a+1)^2 is 0, what is the units digit of (a+2)^2?  Choose the correct option
A) 1.
B) 3.
C) 5.
D) 7.
E) 9.
If the units digit of a^2 is 1, then the units digit of a is either 1 or 9.   If the units digit of (a+1)^2 is 0, then the units digit of a+1 is 0.   To satisfy both conditions, the units digit of a must be 9.   Then a+2 has the units digit of 1, thus the units digit of (a+2)^2 will be 1.   The answer is 1..
<result>A</result>
If a is a positive integer, and if the units digit of a^2 is 1 and the units digit of (a+1)^2 is 0, what is the units digit of (a+2)^2?