From the nature of the carbohydrate bound to red blood cells, answer the following questions: a. People

Question:

From the nature of the carbohydrate bound to red blood cells, answer the following questions:
a. People with type O blood can donate blood to anyone, but they cannot receive blood from everyone. From whom can they not receive blood?
b. People with type AB blood can receive blood from anyone, but they cannot give blood to everyone. To whom can they not give blood?