—I need something for cutting the paper.

— Oh, you want a knife? OK, I'll get ______ for you.

在NH₄NO₃、N₂、NO₂、NO这4种化合物中N元素的化合价有（　　）

古诗词阅读 ，回答相关问题。            
                                                                                     使至塞上
                                                                                        王维
                                                                    单车欲问边，属国过居延。
                                                                    征蓬出汉塞，归雁入胡天。
                                                                    大漠孤烟直，长河落日圆。
                                                                    萧关逢候骑，都护在燕然。

3+6=____

50－10=____

40+40=____

8－8=____