Because of the enormous research and development expenditures required (of firms to survive) in the electronics industry,an industry marked by rapid innovation and volatile demand,such firms tend to be very large. 答案说required of firms 前的that are 被省略了,本来应该是符合sth. be required of sb结构的,但问题是定语从句中不是只有当that作为宾语或表语才能省略吗?这里的that 是主语吗?困惑中。。。