Darlene wrote this proof of the identity (x + y)2 - (x - y)2 = 4xy. which of the following is a justification for step 5 of her proof?

From the given steps:
Step 1: (x + y)2 - (x - y)2 = (x + y)(x + y) - (x - y)(x - y) 
Step 2: (x + y)(x + y) - (x - y)(x - y) = (x2 + xy + xy + y2) - (x2 - xy - xy + y2) 
Step 3: (x2 + xy + xy + y2) - (x2 - xy - xy + y2) = (x2 + 2xy + y2) - (x2 - 2xy + y2) 
Step 4: (x2 + 2xy + y2) - (x2 - 2xy + y2) = x2 + 2xy + y2 - x2 + 2xy - y2 
Step 5: x2 + 2xy + y2 - x2 + 2xy - y2 = 4xy 

The justification for Step 3 of her proof is:

D. Distributive property

What’s the improper fraction?