[文法] 空格內為什麼不能用which

[azide (走開 )]

Pablo Picasso was born on October 25, 1881 to Don JosRuiz Blasco and Do鎙 Ma
ria Picasso Lopez.Pablo spent the first ten years of his life in Malaga, Spain
. The family was far from rich, and when two other children were born, ____ wa
s often difficult to make ends meet.

空格的選項有it what which that，答案是it。請問為什麼不能用which?用it 的原因是
什麼？

--
※ 發信站: 批踢踢實業坊(ptt.cc), 來自: 114.47.68.60
※ 文章網址: https://www.ptt.cc/bbs/Eng-Class/M.1483433883.A.B6D.html
→ sunny1991225: 因為when two other children were born是副詞子句
→ sunny1991225: 後面的it was often difficult才是獨立子句
→ sunny1991225: 你把when two other...前面那個and拆掉，把後半句話
→ sunny1991225: 整個抽出來就很明顯了
→ sunny1991225: (when two other childre were born, it was......
→ azide: 謝謝您