After reading the paragraph below, answer the questions that follow. Researchers have created a robot that has a very thin leg that is moved by cardiac (heart) cells contracting in unison. The robot, made of a polymer similar to that used in making contact lenses, is immersed in a bath of heart cells with supporting cells, which then attach to the robot and provide movement as they contract. Because they are working together in synchrony, the robot's cardiac cells are best classified to the level in life's hierarchy of organization? Group of answer choices organelle organ system tissue organism