It's Sunday today. Sam and his friends are washing(洗)a car for Mr. Green. They can give the money from the car washing to their school football team. Mary and her friends are seeing a funny movie. They see an interesting cat in the movie. Ann is in the park with her mother. They're talking under a big tree. I think they are having a good time. Joan wants to know what her friend is doing. Her friend is in Japan, so Joan is writing a letter to her friend on her computer. May is cleaning the house with her parents. The family are very busy, but they're very happy. They are working and singing.

胚胎干细胞的特点是