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 assigned participants to groups or conditions.
B) made sure that the groups had the exact same number of men and women.
C) made sure that the seizure overheard by participants was identical for everyone.
D) queried participants about their knowledge about epilepsy before beginning the experiment.
E) assigned participants at random to conditions that varied on one key variable.