Q_d=680-9P_x-6I+4P_y

where Q_d=quantity of good X demanded, P_x=price of good X, I=Income, and P_y=price of related good Y. From the demand function, it is apparent that good X is:

I. a normal good

II. an inferior good

III. a substitute for good Y

IV. a complement with good Y

a. II only

b. both I and III

c. both I and IV

d. both II and III

e. both II and IV