She laid a spring mattress under the sheet.

她在床单下面铺了一个弹簧床垫。

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Spring coverage: a spring mattress on the sum of the maximum cross-sectional area measured with the mattress area percentage.

弹簧覆盖率：床垫上所用弹簧的最大截面面积总和与床垫实测面积的百分比值。

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Each berth should be fitted with a comfortable mattress with cushioning bottom or a combined cushioning mattress, including a spring bottom or a spring mattress.

每张床铺应配备带有缓冲底板的舒服床垫或包括弹簧底板或弹簧床绷在内的复合缓冲床垫。

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