Select the statement that expresses an approach used with observational studies.

a$.)"$We took all those notes. We recorded customer counts, menus, food prices, even the general atmosphere of the food truck environment."

b$."$We can test out locations for a brick and mortar by moving our truck. I mean, we have our social media page that gives our daily locations, but what if we found a location we like for a brick and mortar and see if our customers follow us there?"

c$.)$ "Next week, let's send out a coupon for $25\%$ off any menu item to see if we get more customers."

d$.)$ "I think asking questions and collecting the information we want from our population sample is a good start."