When Mr. Baker was declared mentally incompetent, the court appointed Mary, his daughter, to act on his behalf. Several years later, Mary asked her sister, Betty, to manage their father's affiairs. If Betty enters into a listing contract for the sale of their father's home, will the listing contract be valid? A. No, because a contract involving the property of a mentally incompetent person is void. B. Yes, but only if Betty has her father's written consent. C. No, because Betty has not been legally appointed to act on her father's behalf. D. Yes, because Betty has been appointed by Mary to act on behalf of their father