The Correct Option is D. Decrease. If other things being constant, and the supply of Smartphones increase the price will decrease.
In Economics study, the law of demand and supply are fundamental law, which suggest that if there is difference in demand and supply, there will be difference between price of a good.
The Law of demand says that higher prices will lead to less demand, while seller will supply more. And if supply increases while demand is same, the price will fall.
If smartphones supply increases, and the demand rate is same, than the price will decrease. Hence, the correct option is D.
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