My sister gave me a record which is very interesting.从句中做主语的

My sister gave me a record which is very interesting.从句中做主语的是a record吗 那为什么
My sister gave me a record.The record is very interesting.which指代的就是the record 而不是a record了.这仅仅是语法上的需要吗?那究竟which指代的是什么