three tiers合适stratum1.层;地层;(生物的)组织层We can follow the history of life through the strata. 我们可以通过这一层层的地层跟踪生命的历史。 2.社会阶层Tramps are in a low stratum of society. 流浪汉生活在社会的下层。layer 层;阶层;地层A fine layer of dust covers everything. 所有的东西都盖了一层薄薄的灰尘。 floor (楼房的)层I live on the third floor. 我住在三楼。 tier 一层;一层褶裥Her wedding cake had three tiers. 她的结婚蛋糕有三层。
