题干
—What the number of the students in your class ?
—About 60 . A number of them girls .
A：is ; is
B：are ;are
C：is ; are
D：are ; is
答案(点此获取答案解析）
C
—What the number of the students in your class ?
—About 60 . A number of them girls .
A：is ; is
B：are ;are
C：is ; are
D：are ; is
绕原点逆时针旋转
后可得到双曲线
．据此类推可求得双曲线
的焦距为
