Scientific models a. are based on a set of observations b. are used to replace experiments c. allow the study of existing systems only d. are rarely used select the best answer from the choices provided a b c d

i would say C

Explanation:

You can only make a model of something if the real thing already exists.

The correct answer will be option-C.

Explanation:

The scientific models are the simplified visual representations of the complex studied systems of nature.  

The scientific models could be 2 dimensional or 3 dimensional which plays an  important role in scientific studies as they are used by the researchers to prove the validity of their hypothesis if they can be proved or disproved.

They are used by the scientists to explain the complex models to the layman in simple language which helps them able to understand the complex models.

Since the models are used to explain the complex systems which has been discovered therefore Option-C is the correct answer.

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .