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重点讲解：
1. stumble on/upon
意外发现；偶然看见；
eg. History relates that they stumbled on a magnificent waterfall.
历史上记载他们曾意外地发现了一处壮观的瀑布。
eg. I stumble upon the rare book in a second-hand bookstore.
我在一家旧书店偶然找到这本珍贵的书。
2. by chance
偶然；碰巧；
eg. He picked up a purse on his way there by chance.
他碰巧在去那儿的路上捡了一个钱包。
eg. Life gets better not by chance, but by change.
生活变得美好，不是偶然，而是我们努力改变的结果。
3. sit back
不插手；袖手旁观；
eg. They didn't have to do anything except sit back and enjoy life.
他们只需置身事外，享受生活。
eg. They showed me that there are real, wonderful people being treated unjustly and that I cannot sit back and let that happen.
他们让我真实的看到这样一群善良的人受到不平等的对待，而我不能坐视不理。
n. 绊倒，失策
vi. 绊倒，失策，踌躇，无
