Answer is: true.
Demand decreases and competition increases decrease the quantity supplied.
Quantity supplied is the quantity of a commodity that producers are willing to sell at a particular price at a particular point of time.
Quantity demanded is the quantity of a commodity that people are willing to buy at a particular price at a particular point of time.
Quantity demanded can change at the same price depending upon factors like recession, changes in the taste of the consumer.
Competition is rivalry between two or more economic groups.