为你解答.
Playing computer games is bad for our health,but (spending) some time reading is good for us.
句意：玩电脑游戏对健康有害,但花时间读书对我们有益.
解释：填动名词doing,作为句子的主语.可我没见过spend加ing呀有的。而且前一句的句式与后一句完全相同，主语是playing，你自己可以作一下比较。