(B) the demand curve shifts leftward while the supply curve stays the same.
Explanation:
"Substitutes are goods where you can consume one in place of the other. The prices of complementary or substitute goods also shift the demand curve. When the price of a good that complements a good decreases, then the quantity demanded of one increases and the demand for the other increases. When the price of a substitute good decreases, the quantity demanded for that good increases, but the demand for the good that it is being substituted for decreases. "
Reference: Khan Academy. โPrice of Related Products and Demand.โ Khan Academy, Khan Academy, 2019
True
Explanation: