If a '+' sign is assigned to a beneficial interaction, a '-' sign to a detrimental and an 'O' sign to a neutral interaction, then the population interaction represented by '+' '-' refers to

Explanation

Parasitism is an association in which one organism (the parasite) lives on (ectoparasitism) or in (endoparasitism) the body of another organism (host), from which it obtains its nutrients. This association is beneficial for the parasites as they get continuous supply of nutrients from their host and are able to rapidly multiply their numbers. But it is detrimental for the host organism as parasitic infection leads to various complications and diseases in the host body may also be fatal to him under certain circumstances.