Certain Parasites,such as Intestinal Tapeworms,cannot Survive Outside of the Host

Certain parasites,such as intestinal tapeworms,cannot survive outside of the host.Why are they still considered alive?

A) Survival outside of a host is not a requirement for life.
B) Parasites often have life stages that do not require a host.
C) They are still considered alive because they are studied by biologists,and biology is the study of life.
D) As long as they are made of cells,they are considered to be alive.