Some of the steps in the derivation of the quadratic formula are shown. Step3: -c+b^2/4^a=a(x^2+b/a x+b^2/4a^2
Step 4a: -c+b^2/4a=a(x+b/2a)^2
Step 4b: -4ac/4a+b^2/4a=a(x+b/2a)^2

Which best explains or justifies Step 4b?

A. Factoring a polynomial
B. Multiplication property of equality
C. Converting to a common denominator
D. Addition property of equality