Explain whether each of the following events represents (i) a shift of the demand curve or (ii) a movement along the demand curve.

a. A store owner finds that customers are willing to pay more for umbrellas on rainy days.

b. When XYZ Telecom, a long-distance telephone service provider, offered reduced rates on weekends, its volume of weekend calling increased sharply.

c. People buy more long-stem roses the week of Valentine’s Day, even though the prices are higher than at other times during the year.

d. A sharp rise in the price of gasoline leads many commuters to join carpools in order to reduce their gasoline purchases.