rarefied
(of the oxygen level in the air) containing a lower-than-average amount of oxygen
稀薄的
[形容词]
gradation
a series of gradual changes or stages, usually indicating a progression or sequence of steps
层次
[名词]
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(of the oxygen level in the air) containing a lower-than-average amount of oxygen
稀薄的
a series of gradual changes or stages, usually indicating a progression or sequence of steps
层次