Why are descriptive research methods limited in their ability to explain behavior?

Amaster's degree candidate needs to conduct a research project for her master's thesis. she is interested in the types of junk food available to the public. she plans on going to the local convenience stores and asking the owners what types of junk food the store normally stocks and which are the biggest sellers. identifiers will not be collected. this study would fall under which of the following? a. expedited review b. exempt review c. full board review d. not human subjects