题目

There is a new library in our school．It is a high white building．Its large windows are shining in the sun．There are a lot of green trees around it．It looks very beautiful． There are thousands of books in the library. And there are four reading rooms in it．One is for the teachers．The others are for the students．We can read newspapers，magazines and books there． After class, I often go to the library and do some reading．Every Friday afternoon we borrow books from the library．Books are our best friends．We love books．We love our school library，too． 1．The library's 国庆节时，五颜六色的氢气球将节日装扮的靓丽多姿。如图2所示，一氢气球通过软绳与地面上的石块相连，石块质量为m，由于风的作用，使软绳偏离竖直方向，当氢气球和石块相对地面静止时，与石块相连的绳端切线与水平方向成θ角，不计风对石块的作用，则下列说法中正确的是   A．绳子的拉力为mg／sinθ   B．绳子的拉力一定小于mg，否则石块将会被风吹动的氢气球带离地面   C．石块受到地面作用力等于绳子拉力的水平分力   D．石块受到地面作用力不等于绳子拉力的水平分力