What Did Bibb Latané and John Darley (1968)do to Ensure

What did Bibb Latané and John Darley (1968) do to ensure the internal validity of their laboratory experiment on the effects of the number of bystanders on participants' responses to an emergency? They

A) randomly sampled participants for the study.
B) made sure that the groups had the exact same number of men and women.
C) let participants choose what experimental condition they wanted to be in. .
D) asked participants about their knowledge about epilepsy before beginning the experiment.
E) assigned participants at random to conditions that were exactly the same except for one key variable.