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1·Sit with your back straight.

坐着的时候后背要挺直。

2·Keep back straight and bend knees.

背要直，只弯两膝。

3·You should keep your back straight.

你应该把后背挺直。

4·She held her head erect and her back straight.

她昂着头，把背挺得笔直。

5·Our teacher always keeps his back straight.

我们的老师总是把背挺得笔直。

6·Bent your knees, but keep your back straight.

屈膝，保持背部挺直。

7·Sit with back straight, hands at side on floor.

坐姿后背伸直，手放于身体两侧的地面上。

8·Keep your back straight. Thighs tight and shoes right.

保持你的腰背挺直，大腿绷紧和鞋子穿对。

9·Your head should be up, your back straight, and shoulders level.

你的头应该抬起来，你的背部笔直，肩膀水平。

10·Sit or lie down comfortably with your back straight and close your eyes.

找一个舒适的地方躺下或坐下，背部平直，闭上你的眼睛。