Question: Which sentence best describes the organization of the heart?

  Options: 
 A. The heart is an organ made up of different types of tissues, each of which has different types of cells.
 B. The heart is a tissue made up of different types of organs, each of which has different types of cells.
 C. The heart is a system made up of one type of tissue and identical cells.
 D. The heart is a tissue made up of one type of organ and identical cells.
Answer: A