19. 已知直线l1:2x+2y−5=0l_{1}:2x+2y-5=0l1:2x+2y−5=0,l2:4x+ny+1=0l_{2}:4x+ny+1=0l2:4x+ny+1=0,若l1//l2l_{1}//l_{2}l1//l2,则nnn的值为((( )))