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    小题1:   old. I’m studying in a middle school. My school is nice and clean. There are 30 students in my class. It’s    小题2:  today. My classmates and I are    小题3: a good time on the beach. It’s a nice place .The water is blue and clear. Many birds   小题4:   flying over the sea. Many people are    小题5:  in the sea. Some boys are   小题6:   football. Some people are lying   小题7:  the beach. A girl is    小题8:  a book on a chair .Another girl is    小题9:  photos. I   小题10: writing to you! What are you doing?

I hope you can write to me soon.

Best wishes!

Rich

简述农业经济效益形成的特点。