当北京市白昼最长时,太阳光直射的纬线是( )
A:南回归线
B:北回归线
C:北极圈
D:南极圈
已知关于x的方程(m+3)x2+x+m2+2m﹣3=0的一根为0,另一根不为0,则m的值为( )
–– Beijing is a big city, isn't it?
–– Yes. In fact, it's one of ________ cities in the world.