Explain whether the following events would result in a move along an individual's demand curve for popcorn or in a shift of the curve. If the curve would shift, in what direction?
a. An increase in the individual's income
b. A decline in popcorn prices
c. An increase in prices for pretzels
d. A reduction in the amount of butter included in a box of popcorn
e. The presence of long waiting lines to buy popcorn
f. A sales tax on all popcorn purchases?