A, wrong, because if a and b are both negative numbers, the formula =-a-b a-b.

glaring error

C, wrong. If both A and B are negative numbers, then √(a/b)=(√-a)/(√-b).

D, correct √(-a? b？ ) =-ab, because the root sign on the left side of the equation is obviously negative, so if the root sign is meaningful, it must be ab=0.

And then) √(-a? b？ )=0=-ab=0