A3. Which of the following expressions can the expression $\left(x+y+\frac{1}{4}\right)^{2}-\left(x+y-\frac{1}{4}\right)^{2}$ be transformed into?
(A) $4 x y$
(B) $\frac{1}{16}$
(C) $\frac{1}{8}$
(D) 0
(E) $x+y$
A3. We factor the expression using the difference of squares rule and get

$\left(x+y+\frac{1}{4}-\left(x+y-\frac{1}{4}\right)\right)\left(x+y+\frac{1}{4}+\left(x+y-\frac{1}{4}\right)\right)=\frac{1}{2}(2 x+2 y)=x+y$. The given expression can be transformed into the expression $x+y$.