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 bathed in heart cells with supporting cells, which then attach to the robot and provide movement as they contract.
The robot's cardiac cells, working together in synchrony, could be considered at what level in life's hierarchy of organization?
◦ organelle
◦ tissue
◦ organ system
◦ organism