After Cassandra's death, her children discovered that she had sold her house a week prior to her death to her niece, Angelica, for $50,000. Cassandra lived the last ten years of her life in Angelica's house, under her care. The market value of the house at the time of the sale was $500,000. Cassandra's children may attempt to have the sale set aside, arguing that it was the product of ______. fraud undue influence mutual mistake duress