选C：
a^2 - ab - (b^2)/4
=a^2 - ab/2 - ab/2- (b^2)/4
=a(a - b/2) -(b/2)(a - b/2)
=(a - b/2)*(a - b/2)
=(a- b/2)^2
反推：
(a- b/2)^2
=a^2 - 2*a*(b/2) +(b/2)^2
=a的平方 - ab + 四分之一b的平方