Explain whether an experiment or an observational study would be more appropriate to investigate the following:

(a) Whether or not cities with higher unemployment rates tend to have higher crime rates.

(b) Whether a HondaAccord or a Toyota Camry gets better gas mileage.

(c) Whether or not higher college GPAs tend to occur for students who had higher scores on college entrance exams.

(d) Whether or not a special coupon attached to the outside of a catalog makes recipients more likely to order products from a mail-order company.