[請益] 句中 at which的意思

[kirel (Fly dream)]

請問以下句中at which的意思:

it is essential to reform the system for determining the prices at which
the state purchases and sells grain and to take further steps to regulate
the stockpiling of grain.

我的認知是at which=when

因此翻譯為:

    為了決定國家買賣穀物時的價格，改革體系十分重要……

又at which的at是否可省略呢?

謝謝大家


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推 decorum:at which 修飾的是the prices 指國家買賣穀物的價格