题目若单项式2x3ym2x^{3}y^{m}2x3ym和−12y2xn-\frac{1}{2}y^{2}x^{n}−21y2xn的和也是单项式,则mnm^{n}mn的值为( )A.888B.666C.555D.999知识点:合并同类项;单项式乘单项式章节:未标注