Which of the Following Is a Disadvantage of Group Experimental

Which of the following is a disadvantage of group experimental designs that single-system designs may avoid

A) The research design fails to capture the process by which change was induced.
B) It is too time-consuming and costly to secure enough cases for treatment and control groups.
C) Experiments using groups cannot control for threats to internal validity.
D) The research design fails to capture the process by which change was induced, and it is too time-consuming and costly to secure enough cases for treatment and control groups.