[文法] 請教高手一下.....

[macartney (凱薩大帝)]

If human beings had been a bit less greedy and cruel, more birds and animals
__________ dying out.

底線處應為？ 1 ought to avoid
             2 could have been avoided
             3 should have avoided
             4 might have avoided
             5 could avoid

可否順便分析解惑一下  感謝

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推 kurage306:4.憑語感..一定是pp然後3的should意思不對
→ kurage306:咦?那個文法叫pp嗎
推 hankevin:這是與過去事實相反的假設語氣用法 句型如下
→ hankevin:If+主詞+had p.p., 主詞+過去式助動詞+have p.p.
→ hankevin:所以應該是3和4都可以吧 語意上4會好一點
推 jklkj:可以用 would should could might~~~但要依句子而定!!!!!
→ tengharold:4. 3 好像也可，但我不會選3