The heart, liver, and stomach are each examples of
a. a tissue.
b. an organ system.
c. a nerve.
d. an organ.

The correct answer is D. an organ

This is NOT an example of an organ system, as a system implies that they work together for a common goal; your heart has nothing to do with your digestive system.

I got a 100% on my test, so I know this is correct.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

the answer to your question is tooth decay.