当锐角$\alpha >{30}^{°}$时，$\cos \alpha$的值是（   ）

过点A（3，4）的圆的切线方程是   （ ）

对集合{1，5，9，13，17}用描述法来表示，其中正确的一个是（   ）

阅读理解，根据材料内容选择最佳答案。
    The Macintosh family is in England. Mr. Macintosh is a farmer(农场主). He has three people help him on the farm. They help him look after the animals(照顾动物). Mrs. Macintosh is a teacher. She teaches math. She can play the piano and the guitar. Mr. and Mrs. Macintosh have one son and one daughter. Their son's name is Tom and their daughter's name is Fiona. Tom and Fiona like music very much. Tom can play the guitar and his sister, Fiona, can play the piano. Tom and Fiona go to the school music club every Wednesday. Next month they will play for all the teachers and students in their school. Mrs. Macintosh is very happy for them. Mr. Macintosh can't play the guitar or the piano but he can sing. Sometimes, the family plays and sings together(在一起) after dinner in the evening. That's Mr. Macintosh's favorite family activity(活动).

根据图中小磁针N极的指向，标出磁感线方向、通电螺线管的N、S极、并在括号内标出电源的正、负极。