Homework help please:

A study published in 2007 by Christopher Johnson, professor of music education and music therapy at the University of Kansas, revealed that students in elementary schools who took private music lessons scored around 20 percent higher in math scores on standardized tests, compared to students who did not take private music lessons. Is this a randomized experiment or an observational study?

(a) Randomized experiment, since researchers gave students standardized tests.

(b) Observational study, since whether the student took private music lessons was not randomly assigned.

(c) Observational study, since math scores were not randomly assigned.

or (d) Observational study, since this was not a random sample.