阅读下列短文，从每题所给的四个选项（A，B, C和D）中，选出最佳选项。
We are warned not to waste time because time 1 will never return. I think it quite 2. What does time look 3? Nobody knows, and we can't see it or touch it and no 4 of money can buy it. Time is abstract(抽象的), so we have to 5 about it.
Time passes very quickly. Some students say they don't have 6 time to review their lessons. It is 7 they don't know how to make use of their time. They waste it in going to theatres or playing, and 8 other useless things. Why do we study every day? Why do we work? Why do most people 9 take buses instead of walking? The answer is very 10. We wish to save time because time is 11.
Today we are living in the 21st century. We 12 time as life. When a person dies, his life ends. Since life is short, we must 13 our time and energy to our study so that we 14 be able to work and live well in the future. Laziness is the 15 of time, for it not only brings us 16, but also does other 17 to us. If it is necessary for us to do our work today, 18 us do it today and not 19 it until tomorrow. Remember that time is much more 20.
