Which of the following is an example of commensalism?

a. feather lice cling to the hairs of flies and use them for transportation. the flies are not harmed.

b. honeybees gather nectar from flowers. the bees get food and the flowers are pollinated. both organisms benefit.

c. ticks feed off the blood of animals. the ticks get food, but the animals can get sick. only the tick benefits.

d. a virus attacks an organism and feeds off its tissue. the organism gets sick and dies, also killing the virus.