In Attempting to Study Police Officers' Attitudes Toward Their Supervisors

In attempting to study police officers' attitudes toward their supervisors, Alex surveys 25 police officers working in the day shift. In this case, what can help Alex ensure that his findings have external validity?

A) Using a larger sample of officers working in the day shift
B) Randomly assigning officers to work under specific supervisors
C) Surveying officers from all shifts in the department
D) Using confounding variables