Which one of the following is not a threat to the internal validity of an experiment?

Which one of the following is not a threat to the internal validity of an experiment?
 
  a. maturation
  b. regression effect
  c. random error
  d. mortality

Question 2

Jolene just got a 17 out of 100 on a psychology test. Her friend Melba tries to cheer her up by pointing out that she should perform better on the next test, simply due to_____
 
  a. the regression effect
  b. maturation
  c. history
  d. mortality

Question 3

Regression to the mean can occur when
 
  a. performance on two measures is not completely correlated
  b. there is selection on the basis of a measure correlated with the measure of interest
  c. a person who scores on the extremes on one test scores closer to the middle on a later test
  d. All of these are true.