Which of the following is not an advantage of radioimmunoassay？ A: It can detect trace antigen and hapten. B: It can detect trace antibody. C: High sensitivity. D: High specificity.

The basic principle of radioimmunoassay is A: The radiolabeled antigen reacted with excessive specific antibody B: The radiolabeled antigen reacted with a limited number of specific antibodies C: Radiolabeled antigen and unlabeled antigen reacted competitively with limited amount of specific antibody D: Radiolabeled antigen and unlabeled antigen reacted with excessive specific antibody