9．已知A、B是两个单项式，则A＋B一定是

A．单项式                 B．多项式                C．常数                   D．以上都有可能

C．                             D．

A．                          B．

8．下列变形正确的是

C．与                                                 D．与

A．与                                            B．与

7．下列各组是同类项的是

C．                                                 D．

A．                                        B．

6．下列计算正确的是