镁的相对原子质量为24，其意义是(    )

阅读短文，回答问题。

Bear Amy,

   I want to be your friend. Let me introduce my family to you. My father is a doctor. He is tall and strong. My mother is a teacher. She is think and kind. My brother Sam is 6 years old. He's short and funny. We live in a big city. However, we usually go hiking on the weekends. I have a small and nice room. There is a bed and a desk in my room. I like reading books. I have many books on my desk. Welcome to my home.

                                                                                                                                                                        Yours,

                                                                                                                                                                          Lucy

图是世界政区略图，读图并结合材料回答问题。

材料  2011年4月14日，“金砖国家”—中国、俄罗斯、印度、巴西和南非五国在我国海南省三亚举行国家领导人第三次会晤，并签署了金融合作框架协议。

铁工具开始应用于农业生产是在（　　）