Jack Wants to Test If Runners on His Cross-Country Team

Multiple Choice

Jack wants to test if runners on his cross-country team improve with his new training regimen. He takes their times at the beginning of the season and again at the end of the season.
-Runners in Jack's study improving their times due to running generally and not necessarily due to the new regimen is an example of which of the following threats?

A) Selection
B) Maturation
C) History
D) Instrumentation