The part of the conjunctive claim against Defendant C among them is reversed, and this part of the case is remanded.

1. As to this part of the appeal against Defendant B, no grounds of appeal are stated in the petition of appeal or the appellate brief regarding this part of the appeal.

2. As to the appeal against Defendant C (hereinafter “Defendant C”)

A. As to the main claim part of this part of this appeal, no grounds of appeal are stated in the petition of appeal or the appellate brief.

B. Preliminary claim 1) In cases where performance of an obligation becomes impossible after the conclusion of a contract without any cause attributable to both parties, the obligor is exempted from the obligation to perform the obligation, as well as cannot claim any consideration. Thus, contractual relationship is terminated if there was no benefit, and the performance of an obligation already occurred may be claimed for return in accordance with the legal doctrine of unjust enrichment (see, e.g., Supreme Court Decision 2008Da9865, 98662, May 28, 2009). Meanwhile, barring any special circumstance, barring any special circumstance, the obligee is not allowed to seek performance of an obligation, and the performance of an obligation may be claimed for return in accordance with the legal doctrine of unjust enrichment because the performance of the obligation already occurred without any legal ground. Furthermore, the same shall apply to cases where the Plaintiff is unable to perform the obligation under the contract as prescribed in Article 535 of the Civil Act (see, e.g., Supreme Court Decision 2016Da212524, Aug. 29, 2017).