单项选择题
A.par(mfrow =c(2,3))B.par(mfrow =c(3,2))C.par(mfcol =c(2,3))D.par(mfcol =c(3,2))
A.font=B.labels=C.tick=D.at=
A.merge(A ,B ,all.x=T)B.merge(A ,B ,all.y=T)C.merge(A ,B ,all=T)D.cbind(A ,B)