For the vast majority of goods, demand curves slope downward because: a the income effect constitutes almost the entire effect of a price change, and this effect always causes quantity demanded and price to be inversely related. b the vast majority of goods are inferior goods. c marginal utility rises as quantity demanded increases. d the substitution effect constitutes almost the entire effect of a price change, and this effect always causes quantity demanded and price to be inversely related. e the income and substitution effects work in opposite directions.