If any of the alternative subject matters of an obligation becomes impossible to perform,the party having the choice shall not choose the subject matter, unless the performance is made impossible by the other party. 可选择的标的发生不能履行情形的，享有选择权的当事人不得选择不能履行的标的，但是该不能履行的情形是由对方造成的除外。 A joint and several obligation for creditors is an obligation between not less than two creditors the performance of which part or all of the creditors may request from the debtor; and a joint and several obligation for debtors is an obligation between not less than two debtors the whole pertormance of which the creditor may request from part or all of the debtors. 债权人为二人以上，部分或者全部债权人均可以请求债务人履行债务的，为连带债权；债务人二人以上，债权人可以请求部分或者全部债务人履行全部债务的，为连带债务。