When a person must continue to take a drug in order to avoid withdrawal illness, the person has developed 

A) abuse.
B) tolerance.
C) physical dependence.
D) psychological dependence.

Physical dependence occurs when the body becomes accustomed to the drug and needs it to function normally. Withdrawal symptoms occur if the drug is discontinued, leading to a cycle of needing the drug to avoid illness. Abuse and tolerance are related to drug use but do not necessarily indicate physical dependence. Psychological dependence involves a craving or desire for the drug, but not necessarily physical symptoms of withdrawal.