The right answer is C.
Classically positive interactions or facilitators between species are divided into two broad categories:
- those where the two partners involved benefit from each other: the mutualisms (symbolized by + / +)
- those where one of the two partners makes a profit or an advantage while the other gets nothing from it without being affected negatively: it is the commensalism symbolized by + / 0.
It may be that bacterial commensalism has an interest in its host (the animal) in the sense that it protects against parasites, or on the contrary, it favors their appearance.
