— Where is John?
— He is in the playground (操场). He usually _______ at that time.
A:exercise
B:exercises
C:exercising
D:to exercise
东南亚重要的稻米出口国是( )
已知椭圆x2a2+y2b2=1a>b>0的离心率e=12,右焦点为Fc,0,方程ax2+bx-c=0的两个实根x1,x2,则点Px1,x2( )
试赏析《李凭箜篌引》作者写“吴丝蜀桐张高秋”的独特之处。