设f(x，y)具有一阶偏导数，且对任意的(x，y)，都有，则（）．

A ab=<img src="https://www.beizhujiaoyu.com/uploads/202208/162138838040291.png" width="20" height="52" /> B ab=-<img src="https://www.beizhujiaoyu.com/uploads/202208/162138838464591.png" width="20" height="52" /> C ab=0 D ab=2

A <img src="https://www.beizhujiaoyu.com/uploads/202208/162138839998201.png" width="108" height="35" />  B <img src="https://www.beizhujiaoyu.com/uploads/202208/162138840441781.png" width="108" height="34" />  C <img src="https://www.beizhujiaoyu.com/uploads/202208/16213884087301.png" width="166" height="35" />  D <img src="https://www.beizhujiaoyu.com/uploads/202208/162138841224381.png" width="166" height="34" /> 

A <img src="https://www.beizhujiaoyu.com/uploads/202208/162138841796941.png" width="216" height="24" />  B <img src="https://www.beizhujiaoyu.com/uploads/202208/162138842531581.png" width="274" height="26" />  C <img src="https://www.beizhujiaoyu.com/uploads/202208/162138843071801.png" width="231" height="26" />  D <img src="https://www.beizhujiaoyu.com/uploads/202208/162138857250631.png" width="266" height="24" /> 

A Ae^2x+e^2x(Bcos2x+Csin2x) B Axe^2x+e^2x(Bcos2x+Csin 2x) C Ae^2x+xe^2x(Bcos2x+Csin 2x) D Axe^2x+xe^2x(Bcos2x+Csin2x)

A t₀=10 B 150<20 C t₀=25 D t₀>25

A α₁+α₂ B α₂+2α₃ C α₂+α₃ D α₁+2α₂

A A与C相似，B与C相似 B A与C相似，B与C不相似 C A与C不相似，B与C相似 D A与C不相似，B与C不相似