Considering how the body is organized, give one reason why it is so difficult for scientists to build a heart.

A)The heart is a pump made of muscle and that is difficult to grow in the lab.
B)Even if researchers build a heart, there is no guarantee that a person's body will accept it.
C)Human hearts are not like any other animal hearts so there is not source for cells and tissues.
D)The heart is made up of many different types of cells that are organized into different tissues.