Fill in the blanks to make the following statements correct.
a. Economists have designed ______ to better explain and predict the behaviour we observe in the world around us.
b. A variable, such as price or quantity, that is determined within a theory is known as an ______ variable. A variable that is determined outside the theory is known as an ______ variable.
c. When, based on a theory, we claim that "If A occurs, then B will follow," we are making a ______ that can then be tested by ______ observation.
d. If we observe that when variable A decreases, variable B also decreases, we can say that the two variables are ______. We cannot necessarily say that there is a ______ relationship between A and B.