亚洲地势的主要特点是（     ）

根据下列短文内容，在短文后的空格处填上一个恰当的词，使短文完整、通顺。
Dear Bob,
   My name is Rich .I'm fourteen ____  old. I'm studying in a middle school. My school is nice and clean. There are 30 students in my class. It's ____  today. My classmates and I are ____  a good time on the beach. It’s a nice place .The water is blue and clear. Many birds ____  flying over the sea. Many people are ____  in the sea. Some boys are ____   football. Some people are lying____  the beach. A girl is ____  a book on a chair .Another girl is ____  photos. I ____  writing to you! What are you doing?
   I hope you can write to me soon.
   Best wishes!
                                                                                                                                                                                  Rich

朱元璋在洪武十三年(公元1380年)精心设置了一套四辅官制度，以春夏秋冬四季分名其官，春官在春季任事。夏官在夏季任事，同时每季官又设立两到三人分别在上旬、中旬和下旬视事，以此类推。四辅官制度出现的背景最有可能是