Read the passage to answer the question.
Kangaroos are mammals, like humans. Like other mammals, they reproduce sexually with sperm fertilizing an egg that develops into an embryo. Unlike humans, however, once the embryo consumes its yolk sac to develop, it must be born. The infant is
about the size of a lima bean, mostly undeveloped except for its two front arms. It uses these to climb up its mother's abdomen and into her pouch. Once there, it feeds
on its mothers milk and continues to develop until it's big enough to finally leave. The muscle cells of a kangaroo organize together to form the structures that let a
kangaroo walk, run, and hop, allowing it to avoid predators.
Which term names a group of similar cells working together?
tissue
organ
organism
cell system
