Darlene wrote this proof of the identity (x + y)2 - (x - y)2 = 4xy. which of the following is a justification for step 4 of her proof? 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 a. reflexive property b. definition of squaring a binomial c. combining like terms d. distributive property