题目内容
下列计算中正确的有
①-x2n+1yn·3x3ny2n+1=-x5ny3n
②x5n÷(-x3n)=-x2
③x3·=-x6
④(-5x2n+2y3n+1)÷=15xny3n
A.1个
B.2个
C.3个
D.4个