Match the Research Examples in 1-7 with the Main Threat

Match the research examples in 1-7 with the main threat to internal validity that it contains (a-g) .
-An experiment is conducted to assess a new history teaching method. School districts that volunteer serve as the experimental group; those that do not volunteer, serve as the control group.

A) Maturation
B) History or environmental events
C) Repeated testing
D) Instrumentation
E) Selection or assignment
F) Mortality or attrition
G) Statistical regression