How does a vaccine work?
a. it kills viruses.
b. it protects cells from viral infection.
c. it prepares the immune system to recognize viruses.
d. it prevents viruses from being transmitted from an infected person.

Which of these statements describes clinical depression? a. a milder form of depression that usually follows a traumatic event b. a medical condition that may occur due to a chemical imbalance in the brain c. alternate feelings of extreme happiness and extreme sadness