A ab=<img width="18" height="47" src="https://www.beizhujiaoyu.com/uploads/202208/20210520000956_1953.gif" title="" align="" alt="" /> B ab=-<img width="18" height="47" src="https://www.beizhujiaoyu.com/uploads/202208/20210520000956_1953.gif" title="" align="" alt="" /> C ab=0 D ab=2