More than 100 bacterial species live on the surface of the human body. Bacteria cover portions of human skin in concentrations of up to 8 million cells per square centimeter. In particular, human sebaceous glands support the growth of the bacterium Propionibacterium acnes, which uses oil from the glands for food. Two strains of P. acnes are associated with the development of acne on human skin, but other strains are associated with healthy skin. Which statement best describes the relationship between humans and P. acnes?a. In some cases it is commensal and in others it is parasitic.b. In some cases it is mutualistic and in others it is commensalistic.c. It is almost always parasitic.d. It is almost always mutualistic.