Emily received a phone call from her Uncle Old who lives in Florida. He told her he was going to sell her his Kawai grand piano for $8,000 (a good price). Before anything much could be done about all this, Uncle Old died. When Emily told her family about her Uncle's willingness to sell her the piano, they refused to carry out his wishes. Legally, Uncle Old's offer died with him. This is called the termination of an offer via ____ (operation of law / act of the parties).