已知三棱柱$ABC-A_1{B}_1{C}_1$的侧棱与底面垂直，体积为$\frac{9}{4}$，底面是边长为$\sqrt{3}$的正三角形.若$P$为底面$A_1{B}_1{C}_1$的中心，则$PA$与平面$ABC$所成角的大小为(    )

一、二班共有100名学生参加期末体育测试，两班的平均达标率为81%，其中一班的达标率为87.5%，二班的达标率为75%，设一班有学生x名，二班有学生y名，根据题意，可以得到方程组____．

    Christmas is coming. Stephen can see it everywhere he 1 There are Christmas things to sell in the shops and Christmas trees in the park. But it is only in late 2 . Stephen thinks this is wonderful. He likes Christmas very much. To celebrate it for a month is not enough.

     He went into a clothes shop to buy 3 . He found a perfect (完美的) pair.

    "Just these, sir?" asked the shop assistant.

    "Yes, please," 4  Stephen.

    "They look a little big for you. Are you sure they will 5 ?" asked the shop assistant.

    "Oh, they will be fine," said Stephen,

    It was 6 the trousers were a little big. But they  7  to be. Every Christmas Stephen 8 as Father Christmas and works in the local shopping mall. Children like to take photos with him.

    Stephen walked out of the 9 and smiled happily. What 10 he do this Christmas?