plainback

例句：

1. 1. The runner's plainback is smooth and unmarked.
   跑步者的平背光滑无痕。
2. 2. Thechair has a plainback that makes itcomfortable to sit in.
   椅子有一个平背，坐起来很舒服。
3. 3. The plainback of thedress is made from soft fabric.
   裙子的平背是由柔软的布料制成的。