我国优良畜种三河牛、三河马分布的省区是（　　）

以下实验室配制一定溶质质量分数的氯化钠溶液的流程示意图．

请根据示意图回答．

关于的方程$x^2+2x-k=0$有两个相等的实数根，则k的值为（   ）

根据句意和汉语注释，在答卷标有题号的横线上，写出单词的正确形式。
(1)He seldom____  (回复) to the e-mails his friends send him.
(2)Our English teacher has offered us some useful ____ (建议). I think they will help us a lot in our English study.
(3)I came here ____ (专门) to invite you to my party.
(4)Listening to music can make you forget your ____  (悲伤).

如图为电路元件A和B的I一U图象，根据图象提供的信息，求：