氧家族的“生长树”如图1所示．

若⊙O所在平面内一点P到⊙O上的点的最大距离为m，最小距离为n(m>n)，则此圆的半径为(    )

短周期元素W，X，Y和Z的原子序数依次增大．元素W是制备一种高效电池的重要材料，X原子的最外层电子数是内层电子数的2倍，元素Y是地壳中含量最丰富的金属元素，Z原子的最外层电子数是其电子层数的2倍．下列说法错误的是（   ）

把下列各式分解因式:

    Monty was the son of a horse trainer. When he was in his ninth grade, he was asked to write a 1 about what he wanted to be and do when he grew up.
    That night he wrote a seven-page paper saying that someday he'd 2 a horse ranch (牧场). He wrote about his dream in great detail and he 3 drew a picture of a 200-acre ranch with all the buildings.
    He put a great deal of his 4 into the project and the next day he handed it in 5 his teacher. Two days later he received his paper back. On the front page was a large red F with a 6 that read, “See me after class.”
    The boy with the dream went to see the teacher after class and asked, “7 did I receive an F?”
    The teacher said, “This is an 8 dream for a young boy like you. You come from a poor family. Owning a horse ranch requires a lot of 9. You have to buy the land. You have to pay for other things. There is no10 you could ever do it.” Then the teacher added, “11 you rewrite this paper with a more realisticgoal, I will reconsider your grade.”
    The boy went home and thought about 12 long and hard. He asked his father what he should do. His father said, “Look, son, you have to make up your own mind on this. However, I think it is a very important 13 for you.”
    Finally, after sitting with it for a week, the boy turned in the 14paper, making no changes at all. He said to the teacher, “You can keep the F and I'll keep my 15.”