Civil Code, Art. 1205

When the choice has been expressly given to the creditor, the obligation shall cease to be alternative from the day when the selection has been communicated to the debtor. Until then the responsibility of the debtor shall be governed by the following rules: (1) If one of the things is lost through a fortuitous event, he shall perform the obligation by delivering that which the creditor should choose from among the remainder, or that which remains if only one subsists; (2) If the loss of one of the things occurs through the fault of the debtor, the creditor may claim any of those subsisting, or the price of that which, through the fault of the former, has disappeared, with a right to damages; (3) If all the things are lost through the fault of the debtor, the choice by the creditor shall fall upon the price of any one of them, also with indemnity for damages. The same rules shall be applied to obligations to do or not to do in case one, some or all of the prestations should become impossible. (1136a)