dropped,knocked ,covered ,stopped中ed发音不同的是

dropped,knocked ,covered ,stopped中ed发音不同的是 Knocked dropped dropped knocked at knocked是什么意思 knocked-up Just Dropped cov(x,y)=? 概率论COV(2, 协方差cov(x,-y)= -cov(x,y)吗 为什么协方差cov(x,-y)= -cov(x,y) 英语knocked.started.英语knocked.started. be knocked down 赛车中Knocked knoced at 和knocked get knocked out? knocked什么意思