Concepts are the human brain's understanding of the essential characteristics of objective things, and the essential characteristics of things are the characteristics that determine the nature of things and distinguish one thing from other things. According to the degree of abstraction and generalization of the attributes contained in concepts, concepts can be divided Concrete concepts and abstract concepts. Concepts formed according to the designated attributes of things are called concrete concepts, and concepts formed according to the intrinsic properties of things are called abstract concepts. Which of the following is an abstract concept. ['Bananas and sticks are long, while balls and watermelons are round.', 'Bananas and watermelons are fruits, cabbages and potatoes are vegetables.', 'Chocolate and land are brown, snow and flour are white.', 'Ice and snow are cold, fire and blood are hot.']