In Examining Threats to Internal Validity History, Selection Bias and Testing

In examining threats to internal validity history, selection bias and testing are described below. Which of the following is NOT a description of those threats?

A) Events outside the program differentially affect the scores of the experimental or comparison group.
B) Differences in the dropout rate between the experimental group and the comparison group differentially affect effect the scores between the two groups.
C) Members of the experimental and comparison groups are not truly equal on all important factors.
D) The pretest differentially affected the learning of the experimental or comparison group.