句型转换 根据括号内的要求改写句子，在答题卷相应的横线上填写答案，每空限填一词。（本大题共5分，每格0.5分）
My mother has already finished her housework. (改为否定句 )
My mother ________ finished her housework ________.
The factory put its waste into the river in the past. (写出同义句)
The factory ________ ________ put its waste into the river.
The boy is young. He can’t go to school. (两句合并成一句)
He is ________ young ________ go to school.
Mr. Wang has taught in this school since he graduated from college. (划线部分提问 )
________ ________ has Mr. Wang taught in this school?
The problem is too difficult for me to work out. (写出同义句)
The problem isn’t ________ ________ for me to work out.