In an insect,

a. nutrients are circulated through an open circulatory system.
b. the blood vessels do not form a continuous circuit
c. the fluid flowing through the heart is the same fluid that bathes the tissues.
d. the circulatory system is responsible for exchange of respiratory gases

In an insect, A. nutrients are circulated through an open circulatory system.

Explanation:

Insect respiration is independent of its circulatory system, that's way the blood does not play a direct role in oxygen transport and the circulatory system is responsible for exchange of respiratory gases. Insects also have an open circulatory system as opposed to our closed circulatory system.

it's anything that aren't producers and consumers.

explanation:

you've got to be clear on what "this" is

carbon atoms, hydrogen, oxygen, and nitrogen.