A:Playing computer games.
B:Listening to music.
C:Watching birds.
— ________ is this schoolbag?
— Thirty-five yuan.
如图,内外两个椭圆的离心率相同,从外层椭圆顶点向内层椭圆引切线AC,BD,设内层椭圆方程为x2a2+y2b2=1a>b>0 ,若直线AC与BD的斜率之积为-14 ,则椭圆的离心率为( )
历史课外兴趣小组的同学准备办一期黑板报,宣传第二次工业革命,下列适合的标题是( )