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同学你好~
是的，这里which had been dyed black是非限定性定语从句。
这里which指的是wool，之所以用非限定性定语从句，是因为which had been dyed black这部分信息是对wool的一个补充说明，去掉也不影响句子的理解。
Its wool, which had been dyed black, had been washed clean by the rain!  这里不能用and连接
去掉which引导的从句，主句是Its wool had been washed clean by the rain. 主语its wool和谓语had been washed之间不需要and。



限定性定语从句和非限定性定语从句的区别：

最直观的区别是限定性定语从句不用逗号和主句隔开，而非限定性定语从句要隔开
Our guide，who was a French Canadian，was an excellent cook.
我们的向导，一个法裔加拿大人，擅长于烹调。
Ocean currents affect the climates of the lands near which they flow.
洋流影响其流经的附近地区的气候。

限定性定语从句用于对先行词的意义进行修饰、限制和识别，如果去掉，就会造成句意不完整或概念不清；而非限定性定语从句用于对先行词起补充说明作用，如果省略，句意仍然清楚、完整： People who take physical exercise live longer.
进行体育锻炼的人活得长些。(若把从句去掉句子就失去意义)

His daughter, who is in Boston now, is coming home next week. 他女儿现在波士顿，下星期回来。(若把从句去句子意义仍然完整)


限定性定语从句的先行词只能是名词或代词，而非限定性定语从句的先行词则可以是名词或代词，也可以是短语或句子
He changed his mind, which made me very angry. 他改变了主意，这使我很生气。(which指整个主句)

祝同学学习进步~