Sigmund enters into a contract with Carl.The terms are that Sigmund will purchase all the gasoline that he wants to purchase in 2011 at a price of $2.50 per gallon,and Carl agrees to sell on those terms.This is an example of a(n) :

A) valid contract.
B) illusory contract.
C) voidable contract.
D) implied contract.

For a promise to serve as consideration in a bilateral contract,the promisee must have promised to do something at the promisor's request.It is obvious that if the promisee's promise is illusory because it really does not bind the promisee to do anything,such a promise could not serve as consideration.Such agreements are often said to lack the mutuality of obligation required for an agreement to be enforceable.