Many things now cost double what they did a year ago.中what they did a year ago是什么成分？

我语法虽然不太好

但是 我觉得 这一句 what they did a year ago 是从句,修饰 double,而double 修饰的是 cost 作谓语.所以 这句 应该是作 谓语 修饰动词 cost.

是有many things引导的定语从句，修饰many things的，意思是他们一年前做的事情现在的成本翻了一倍。