For each of the following shifts in the demand curve and associated price change of a complement or substitute item, explain whether the price of the complement or substitute must have increased or a decreased.
a. A rise in the demand for a dashboard global-positioning-system device follows a change in the price of automobiles, which are complements.
b. A fall in the demand for e-book readers follows a change in the price of e-books, which are complements.
c. A rise in the demand for tablet devices follows a change in the price of ultrathin laptop computers, which are substitutes.
d. A fall in the demand for physical books follows a change in the price of e-books, which are substitutes.