The small white insects (cochineal) that live in the arroyo seco, mostly on top of the beaver tail cactus and sucks sap and water from the cactus, without providing anything for the cactus in return, is an example of

Parasitism

Explanation:

The parasitism is a type of biological relationship in which the one organism depends on the other organism for its need and in turn the organism does not get benefits from it.

Means out of two only one organism is benefited in this type of a relationship. The insect sitting on cactus sucks the sap from it and does not provides any benefit to the plant.

The plants gets affected by the insects and there is no profit of plant in this case.

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.