阅读短文,下列问题

    Today is Saturday. It is Children's Day, I get up very early. I go to school at about six. Our school is very cleanand beautiful. Birds in the trees sing all the time. Every student wears a white coat. Girls wears beautiful skirts. We sing and dance, run and jump. We look like happy birds.

    Our party begins at half past seven. First, we listen to our teachers, and then the programme(节目) begins. Many people sing together on the stage(舞台) . They sing and dance very well. All the teachers and students like watching and listening to them. After the party we play games in the classroom. All of us enjoy the day very much.

正方体ABCD﹣A′B′C′D′中，AB的中点为M，DD′的中点为N，则异面直线B′M与CN所成角的大小为（　　）

同学们浇树，每个人浇4棵，2个人一共浇8棵。

亚洲东部和南部易发生旱涝灾害，主要与下列哪一因素的影响有关