Question

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, so steve bought two hamburgers and a soda. steve's response to the decrease in the price of hamburgers is best explained by: the substitution effect. the income effect. the price effect. a rightward shift in the demand curve for hamburgers.

Answer 1

The answer is The income effect. 
Income effect is described as the change in demand of a service or good brought on by change in the income of a consumer.It is observed in two cases first is when income of person increases and second is when price of goods or service decreases. 
The scenario given in the question is an example of second case as the price of burger was less than normal Steve perceived his income to be able to buy more product in same price