新概念三冊People who live in Britain  needn't despair when they made mistakes like this (and a lot of  people do)
I had been asked to drive in heavy traffic and had done so successfully.

新概念二冊The snake then wound itself round the wires. When it did so, it sent sparks down to the ground and these immediately started a fire.

英語中會用so指代前面的事情，后面兩個例子都用了SO，為什么第一個沒有用呢？