【单选题】Activin is a protein that normally exists in a gradient within certain tissues in developing embryos. Certain undifferentiated cells cultured without activin produce epidermal cells. However, activin ...
【简答题】完形填空。 Last January I was given a chance to work in another country. At first, I lived a 1 life, but soon I met a group of new friends and they were really nice. Then something 2 happened. One of our f...