With the loss of labor，the old farmers ________at home had to choose less laborintensive crops.
A：having left
B：to leave
C：left
D：leaving
— Listen! Someone is knocking at the door. Who can it be?
— It _______ be Tom. He said he would visit me this evening.
为了这个家，我们的父母付出了很多____、____、____。
下列方程中是关于x的一元二次方程的是（ ）
对下列句中破折号的用法解释有误的一项是（）
