Left: spiral-arm patterns traced by classical Cepheids of different ages under the four-arm model. Right: spiral-arm patterns traced by classical Cepheids of different ages under the multiple-arm model. Top: subsample A; bottom: subsample B. White dashed lines show maser-traced spiral arms; white triangles denote masers. Colored circles represent classical Cepheids (filled: with v_LSR; open: without), with colors indicating assigned spiral arms (see Figure 2). The solid lines represent the best-fitting spiral arms traced by classical Cepheids, with dashed lines indicating their widths.