阅读下面短文，从所给的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项。
Once, a professor held a test on his students. The professor started with the following question, “Now,1 you are going to cut trees on a hill. One of the trees has a big 2 while the other's small. Which will you cut?” The question got a quick 3, “Of course the big one! No need to4 .”
The professor5, “The big tree is a poplar（白杨）6seen everywhere, while the small one is a Korean pine. Consider again?” A second7brought students more sense: a Korean pine is much more8than the poplar, so they changed their decision. “9 we'll cut the pine. The poplar isn't worth our10.” Smile still11on the professor's face. “But the pine's trunk is less12than the poplar.” Rather puzzled, they became13, then said, “While, we'll cut the poplar.” The professor gave more detail. “14the poplar is straight, the trunk became empty within. Then what's your choice?”
Finally one of the students lost his15and said, “Professor, you have asked a series of questions leading us to change our answers16. What do you want to 17 after all?”
The professor's smile18. He said, “Why none of you ask me for what purpose to cut the tree? Though my questions 19all the way, they are decided by one thing: your20to take the action. If it aims at making handicraft articles you may cut the pine; if you just want to have some firewood, you'd better cut the poplar.”
