Question

I just want to check my answers. Help go over them with me please to make sure they're right. Thank you.

In which of the following cases what an observational study be necessary as compared to an experiment study?

A. the study of how increased nutrient levels affect plant growth.
B. the study of how educational levels affect median household income.
C. the study of how a vaccine affects the percent of mice that get a particular disease.
D. the study of how noise level affects the sleep patterns of volunteers in a sleep study.

My answer was B.