The Parkwood School District $($PSD$)$ contracts with Creamy Dairy Farms for "all of the milk" Creamy Dairy Farms can produce during PSD$\text{'}$s current school year. The contract specifies a price, but no exact quantity of milk to be purchased is set in the contract. This contract is a$($n$)$

invalid contract because no quantity is indicated

valid requirements contract

valid output contract

invalid contract because consideration is missing