They have been waiting for many hours to see the singer, but the airplane must have been _____.

Mary is ill today. If she ______ ill, she _______ absent from school.

Only after a baby seal is pushed into the sea by its mother _____ to swim.

What do you think of the game called “trick or treat”—_______________.

填入下面一段文字横线处的语句，最恰当的一句是（ ）对于每个学生来说，在进行升学规划时， 也是促进学校办出高质量的动力所在。