天净沙·秋思                       天净沙·春

                                                                             （元）马致远                         （元）白朴

                                                                               枯藤老树昏鸦，                    春山暖日和风，

                                                                               小桥流水人家，                    阑干楼阁帘栊①，

                                                                               古道西风瘦马。                    杨柳秋千院中。

                                                                               夕阳西下，                           啼莺舞燕，

                                                                               断肠人在天涯。                    小桥流水飞红②。

【注】①帘栊：带帘子的窗户。②飞红：花瓣飞舞，指落花。

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在代数式2xy²，﹣x，3，x+1，ab﹣x²，2x²﹣x+3中，是单项式的有（　　）