These paintings which were painted by a famous artist cost a lot of money.
首先这句话的意思是这些画是一位著名画家画的，花了很多钱
那么这里的空就应该是形容画的，形容物品的话是用which，人的话是用who
前面these paintings表示的是复数，那么就应该是were而不是was
分拆原句：
① These paintings cost a lot of money.
② These paintings were painted by a famous artist.
那么当②做定语修饰①的主语时，②的 these paintings 替换为定语从句先行词，物用 which。
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