严格说来，填 was 或 were 都可以，但以 were 较为普通。一般说来，a mass of...作主语时谓语动词通常与其后所接名词的数保持一致，即 a mass of 后接单数名词时连用单数动词，后接复数名词时连用复数动词。又如：
A mass of snow lies in the yard. 院子里积了大量的雪。
A mass of customers are entering the supermarket. 大批顾客正走进那家商场。
但有时 a mass of 后面接复数名词时也可能接单数动词，即把 mass 视为主语。如：
A mass of people was / were standing before the courthouse. 一大群人站在法庭外。
