题干
While we _______ football in the classroom, the teacher came in.
A：were play
B：were playing
C：are playing
D：played
答案(点此获取答案解析）
B
While we _______ football in the classroom, the teacher came in.
A：were play
B：were playing
C：are playing
D：played
”是“方程
”表示焦点在y轴上的椭圆”的（ ）