Tankfield emailed Pea some photos, which Pea did not find interesting.
which就近指代photos，photos是定語從句中的賓語，把定語從句還原成一個完整句就是Pea did not find the photos interesting.
可以看到，上面兩個句子都可以以that為關系代詞改寫，即
(1)Tankfield emailed Pea some photos that did not cheer Pea up.
(2)Tankfield emailed Pea some photos that Pea did not find interesting.
which前面是介詞，介詞前面是名詞，which就近指代介詞前的那個名詞，名詞和介詞之間有無逗號均可。
Tankfield once lent Pea a book from which Pea glimpsed the basics of genes.
這個句子可以“拆”成如下兩句：（每個定語從句都可以拆成兩句，以先行詞為拆分點）
(1)Tankfield once lent Pea a book.
(2)Pea glimpsed the basics of genes from the book.
先行詞實際上是定語從句中介詞的賓語，這時，需要把介詞提前，避免如下表達：
(1)Tankfield once lent Pea a book, which Pea glimpsed the basics of genes from. (XXX)
(2)Tankfield once lent Pea a book that Pea glimpsed the basics of genes from. (XXX)
上中學的時候學過，which引導的定語從句叫“非限制性定語從句”，是不是？它怎么就“非限制性”了？回顧上述GMAT語法規定which有且僅有的兩種用法，which指的都是就近的名詞，從句其實還是限制性的。
所謂非限制性呢，是說which指逗號之前一套主謂所描述的情況，以逗號前的一個完整句為先行詞。比如：
(1)Tankfield might never care to say Happy Birthday to Pea, which was rather sad.
(2)Tankfield might never care to say Happy Birthday to Pea, which Pea did not refuse to acknowledge to herself any more.