Steve went to his favorite hamburger restaurant with $3, expecting to buy a $2 hamburger and a $1 soda. When he arrived he discovered that hamburgers were on sale for $1 each, so Steve bought two hamburgers and a soda. Steve's response to the decrease in the price of hamburgers is best explained by:

A. the substitution effect.

B. the income effect.

C. the price effect.

D. a rightward shift in the demand curve for hamburgers.