1, and 2 are the answer and by one and 2 I mean the first 2 statements
The cells in the heart need a constant supply of nutrients / oxygen and to get rid of their waste products ans that why it needs its own special circulatory system.
A. A UNIT OF ENERGY SUPPLIED BY FOOD.
A calorie is a unit of energy. Historically, scientists have defined "calorie" to mean a unit of energy or heat that could come from a variety of sources, such as coal or gas. In a nutritional sense, all types of food — whether they are fats, proteins, carbohydrates or sugars — are important sources of calories, which people need to live and function.
