Metro Facilities, Inc, contracts to sell a parking garage to Nouveau Property Company. The contract pro¬vides that if Metro does not close the deal by September 15, it must pay Nouveau one-half of the contract price. This provision isnot enforceable because it is

a. a liquidated damages clause.
b. a waiver of breach clause.
c. a limitation-of-liability clause.
d. a penalty clause.

D