I'm pretty sure the answer is a. but anybody feel free to correct me :)
a. how many species either benefit or are harmed...
The main difference among types of symbiotic relationships lies in how many species either benefit or are harmed. Mutualism has both species benefiting, commensalism has one benefiting without harm to the other, and parasitism has one species benefiting at the cost of the other. Therefore, the correct answer is option a.
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