A leftward shift in the supply curve indicates a decrease in supply. This can be caused by:
- A decrease in the number of sellers. When there are fewer sellers in the market, the overall supply of the good or service decreases, leading to a leftward shift in the supply curve.
The other options would typically not cause a leftward shift in the supply curve:
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Increased demand due to advertising would generally shift the demand curve to the right, not the supply curve.
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A decrease in input prices would typically lead to an increase in supply, shifting the supply curve to the right.
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A technological improvement usually makes production more efficient, which would also increase supply and shift the supply curve to the right.
Therefore, the correct answer is A decrease in the number of sellers.