When running a gel you need to have a positive control and negative control. what do these mean and what are they each used for?

I believe the positive control receives a treatment with a known response, so that this positive response can be compared to the unknown response of the treatment. This is used in electrophoresis to compare the DNA strands to the DNA standard. The negative control is used when no response is expected, which is also used in the process of electrophoresis. Electrophoresis is a technique used in laboratories  in order to separate macromolecules based on size. It applies a negative charge so proteins move towards a positive charge. It is sued for the analysis of both DNA and RNA. 

Ithink the answer is d.