Underline the topic sentence of each paragraph.
1. Since computers were invented in the USA in the 1950s, they have become very important in many areas of work and leisure.
2. Since the Internet came into being, people's lives at home have been changed, too.
3. Computers have improved our lives, but they have brought problems, too.

1. For instance, if we work on computers too long,
we may get head aches or sore eyes. 比如:如果我们使用的电脑工作的时间太长，我们就会头昏眼花。
for instance =for example用来举例说明某一论点或情况，一般只举同类人或物中的“一个”为例。
such as用来列举事物时，一般列举同类人或事物中的几个例子。E.g.
(1) Many countries, Japan, for instance (for example), has a lot of earthquakes.
(2) I like fruits, such as apples, pears and bananas.