What could be the rationale behind using multidrug therapy instead of single drug therapy?

What could be the rationale behind using multidrug therapy instead of single drug therapy? B. What would happen to the virus population if multiple drugs are given to a patient in a sequence (not simultaneously) over course of few months? C. What would happen to the virus population, if multiple drugs are given simultaneously? D. What would happen to the viral population if the patient is taken off the drugs for a while as a way of helping the patient to cope with side effects of the drugs?