若a>0,a≠1,x>0,y>0,x>y,下列式子中正确的个数有( )①logax?logay=loga(x+y);②logax-logay=loga(x-y);③logaxy=logax÷logay;④loga(xy)=logax?logay.A.0个B.1个C.2个D.3个