Assume the demand for popcorn is downward sloping. Explain whether the following events would result in a move along an individual's demand curve or would cause a shift of the curve - and if it would cause a shift, provide the direction of the shift and a brief explanation of it.
a. An increase in income
b. A reduction in the amount of butter included in a box of popcorn
c. A sales tax on all popcorn purchases
d. An increase in the price of pretzels
e. A decline in the price of popcorn.