Answer:
a change in buyer tastes.
Explanation:
Increase in demand for doughnut can be caused by:
1. Change in buyers tastes: if the taste of consumers change in favour of doughnut, the demand for doughnut increases.
2. Increase in the price of substitutes: if the price of a substituite e.g. bagel increases, the demand for doughnut increases because consumers would subsituite a bagel for doughnut. Coffee can't be considered as a subsituite for doughnut.
3. Increase in income : if income increases, demand for doughnut increases.
Increase in cost of production reduces supply and doesn't affect demand.
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