a portion of the quadratic formula proof is shown. fill in the missing reason.

ax^2+bx+c=0 —— Given
ax^2+bx=-c ——subtract c from both sides of the equation
x^2+b/a x=-c/a —— divide both sides of the equation by a
x^2+ b/a x+(b/2a)^2=-c/a+(b/2a)^2 —— complete the square and add (b/2a)^2 to both sides.
x^2+ b/a x+(b/2a)^2=-c/a+b^2/4a^2 —— square (b/2a) on the right side of the equation.
x^2+ b/a x+(b/2a)^2=-4ac/4a+b^2/4a^2—— find a common denominator on the right side of the equation.
x^2+ b/a x+(b/2a)^2=b^2-4ac/4a^2 —— ?????

A-multiply the fractions together on the right side of the equation
B- subtract 4ac on the right side of the equation
C- add 4ac to both sides of the equation
D- add the fractions together on the right side of the equation