To Examine the Effectiveness of Empathy Training on Children's Sharing

To examine the effectiveness of empathy training on children's sharing behavior, two months of training are provided to 3-year-old children living in rural Georgia. Six months later, the children's sharing behaviors are measured and compared to their pretraining sharing behavior. Improvements are considered the result of the empathy training. Which of the following properties is a potential source of internal invalidity?

A) All children were from rural Georgia.
B) Children's sharing behavior may improve with the simple passage of time.
C) The observers of the sharing behavior may disagree.
D) Children who were the best sharers before the training were not necessarily the best after the training.