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

To examine the effectiveness of empathy training on children's sharing behaviour, 2 months of training are provided to 3-year-old children living in rural Georgia. Six months later, the children's sharing behaviours are measured and compared to their pretraining sharing behaviour. Improvements are considered the result of the empathy training. Which factor is a potential source of internal invalidity?

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