重点讲解：
1. stumble upon 意外发现；偶然看见；
例句：It had surprised him to stumble upon a dirty, forgotten place like that in a bank that was always so tidy.
在像银行这样一个素来十分洁净的地方，竟会撞见一处肮脏的，被人遗忘的死角，使他感到十分惊讶Go4!A5bv3+J@6TW@d4B。
2. be known as 称为；叫做；
例句：This process is known as adaptation.
这个过程即所谓适应hZ.Ia)(ykEpJbQv。
3. hold up （论点、理论等）经受得住检验；
例句：I'm not sure if the argument holds up, but it's stimulating.
我不能肯定该论点是否经得起检验，但它很有新意zNjpzQ5]|8|Bu-qA9@_。
4. in theory 从理论上说；照理说；
例句：That may be reasonable in theory but it sounds fanciful in practice.
从理论上看这是说得过去的，可实际上却有点异想天开UIh&NvCx[D。