题目今天数学课上,老师讲了单项式乘多项式,放学回到家,小明拿出课堂笔记复习,发现一道题:−7xy(2y−x−3)=−14xy2+7x2y□-7xy\left(2y-x-3\right)=-14xy^{2}+7x^{2}y\square−7xy(2y−x−3)=−14xy2+7x2y□,□\square□的地方被钢笔水弄污了,你认为□\square□内应填写（ ）A.+21xy+21xy+21xyB.−21xy-21xy−21xyC.−3-3−3D.−10xy-10xy−10xy知识点：单项式乘多项式章节：未标注
