A Park Ranger at Shenandoah National Park Wants to Know

Multiple Choice

A park ranger at Shenandoah National park wants to know if visitors to our country's national parks can properly identify poison ivy. Over the course of a month, she shows several plant varieties to 30 randomly selected visitors per week, and 74 of them correctly identify the poison ivy. The biggest potential weakness of experiments is that:

A) experiments do not give good evidence for cause and effect.
B) experiments only work when one can give a placebo.
C) it can be hard to generalize conclusions beyond the actual subjects to a wider population.
D) informed consent is often not possible.