A、2x-2y
B、2xdx-2ydy
C、<img width="92" height="24" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/c76b761c-0227-4356-92a4-3af3a9ab41ba.png" />
D、<img width="157" height="24" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/d687e424-d18d-4a97-b241-efc0f216b7e9.png" />
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