Some of the steps in the derivation of the quadratic formula are shown.

Step 3: –c + StartFraction b squared Over 4 a EndFraction = a(x squared + StartFraction b Over a EndFraction x + StartFraction b squared Over 4 a squared EndFraction)

Step 4a: –c + StartFraction b squared Over 4 a EndFraction = a(x + StartFraction b Over 2 a EndFraction) squared

Step 4b: negative StartFraction 4 a c Over 4 a EndFraction + StartFraction b squared Over 4 a EndFraction = a(x + StartFraction b Over 2 a EndFraction) squared

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