Parasitism is when

• A. One species benefits while the other species is harmed.
• B. Both species benefit from the interaction.
• C. Neither species benefits.
• D. The relationship is always harmful to both.

Answer: (A)

Parasitism is a relationship in which one organism—the parasite—lives on or inside another organism—the host—and benefits by obtaining nutrients or resources from the host. The host, on the other hand, is harmed, though not always killed quickly; the harm can include reduced health, growth, or reproduction. This idea fits because the parasite gains a resource without giving one in return, while the host bears the cost of that extraction. This differs from mutualism, where both organisms gain, and from commensalism, where one benefits but the other is largely unaffected. It’s also not about one organism killing the other for food, as in predation, where the relationship ends with the predator consuming the prey. So the best description is a one-sided benefit with harm to the other, which is exactly what parasitism describes.