根据短文理解选择正确答案。

      A good way to pass an exam is to work hard every day in a year. You may fail （不及格）in the exam if you are lazy for most of the year and then work hard only a few days before the exam. If you are taking an English exam, do not only learn rules of grammar. Try to read stories in English. Speak in English whenever（无论何时）you can. A few days before the exam you should go to bed early. Do not stay up late at night studying. Before you start the exam, read carefully over the question paper（试卷）, try to understand the exact(确切的) meaning of each question. When you have finished your exam, read over your answers. Correct(改正) the mistakes if there are any, and be sure that you have not missed anything out.

分母是2的所有真分数的和是 $\frac{1}{2}$ ；分母是3的所有真分数的和是 $\frac{1}{3}$ + $\frac{2}{3}$ =1，可以写成 $\frac{2}{2}$ ；分母是4的所有真分数的和是 $\frac{1}{4}$ + $\frac{2}{4}$ + $\frac{3}{4}$ =$\frac{3}{2}$；分母是5的呢?请你认真思考填写下表。

分母是90的所有真分数的和是____。

2016年国庆黄金周期间，家住沭阳的小王打算去北京旅游，为了在旅游期间便于购物消费，增强交易安全，除了带一些零花钱之外，以下最适合他携带的结算工具是（  ）

明朝中后期，中国出现了资本主义萌芽，是在商品经济发达的​（   ）