Question

parasitic relationships are often the most familiar types of symbiotic relationships. a common example is a flea living on a dog. in cases like these, the parasite gains a viable habitat to support its life cycle, while the host is harmed.
in a mutualistic relationship, on the other hand, both organisms in the relationship are better off because of their arrangement; that is, they benefit mutually from their interactions. for example, lichen is composed of two organisms, an alga and a fungus. the fungus acts like a sponge, holding on to water that the alga needs. at the same time, the alga is constantly converting sunlight into energy, so the fungus, which cannot perform this function for itself, benefits from a reliable energy source. lichen is, in turn, engaged in a commensalistic relationship with the tree that it grows on. in a commensalistic relationship, one organism benefits and the other organism neither benefits nor suffers. the tree provides lichen with a place to grow, but the tree is unaffected.
what is the meaning of host as used in the passage?
an organism that lives in or on another organism
an organism that relies on the sun for food
an organism that another organism lives on

Explanation:

To determine the meaning of "host", we analyze the context. The passage says in a parasitic relationship (e.g., flea on a dog), the parasite gains a habitat and the host is harmed. Let's evaluate each option:

• Option 1: "an organism that lives in or on another organism" describes a parasite (like the flea), not the host. Eliminate.
• Option 2: "an organism that relies on the sun for food" refers to the alga (from the mutualistic example), not related to the host definition. Eliminate.
• Option 3: "an organism that another organism lives on" matches the host's role (e.g., the dog for the flea, the tree for lichen in commensalism), as the host provides a place for the other organism (parasite/commensal) to live and is harmed (in parasitism) or unaffected (in commensalism).

Answer:

an organism that another organism lives on