Salt is the answer to all of the questions stated above. Salt has been an important spice in history. Salt is the tiny crystal that filled the coffers of the empire. Salt is the mineral that has the power to destroy or to create. And, it is salt which is 'good as gold' when in scarcity and 'cheap as dirt' when in abundance.
Salt has a history of its own. It historical times, salt was used as a means of economic trade and currency. In Egyptian culture, salt was used as an offering. Today, we have salt in abundance and hence just 'cheap as dirt' but in history, there was a time when even the production of salt had restrictions. It was 'good as gold' in historic times.
The reason is because a simile has like or as in the sentence and a metaphor is referring to an object in the sentence and a rhetorical question is asking a question so those are wrong.
