A flea and dog are an example of Choose... , because the flea irritates the dog's skin and feeds on its blood. Certain bacteria live in the digestive tract of humans, and their relationship is an example of Choose... because the bacteria help humans to break down the food they eat. Cattle egrets eat insects off the back of an elephant, which is an example of Choose... because the cattle egrets benefit without either harming or helping the elephants.

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A flea and dog are an example of parasitism, because the flea irritates the dog's skin and feeds on its blood.

In a parasitic interaction, one organism benefits whereby the other one harmed. It is a positive - negative relationship. This interaction involves the interaction between the host and the parasite. The host is the bigger organism, here, the dog that is being harmed. The parasite is the flea causing the harm and benefiting from the relationship.

Certain bacteria live in the digestive tract of humans, and their relationship is an example of because mutualism the bacteria help humans to break down the food they eat.

In mutualism, the interaction is based on both organism deriving benefit from one another. Here, the bacteria gets its nourishment by breaking down human food. Human beings are able to digest the food better and are also nourished. A win-win situation is established.

Cattle egrets eat insects off the back of an elephant, which is an example of commensalism because the cattle egrets benefit without either harming or helping the elephants.

This relationship is known as commensalism in which one organism derives benefit. The other one derives no benefit and it is also not harmed in the process. The egrets feeds on the insects that are not harming the cattle.

Explanation:

the answer is b.) hydrogen bonds between bases.