Reading讲一个explosion，说这个explosion不是由asteroid 掉下来引起的. 举了三点:
A．2周爆炸后去找particle是可以找到了。（某explosion的例子，reading中用来doubt和compare）但是Tunskga explosion 后19年人们才去search.，那么particles很有可能被河水和融化的冰冲走。另外，发现当时河上的确是有ice的。所以可能性更大。
B．如果asteroid explosion 都要有crater的话，人类就没法活了。掉一颗就毁了一地，人家asteroid 里面有iron，没听过asteroid 会在经过大气层以后燃烧么，只有一小部分还烧剩一点,砸到地球上变成crater.
C．物理化学没学好是吧，trees after the explosion show the force came out like asteroid explosion.而且crater pattern也可以证明不是gas explosion.该pattern 是concerntrated gas不可能形成pot。
Grades encourage students to learn
Learning is by no means an easy thing, so people often think up various ways to enhance students’ studying efficiency and stimulate students’ studying interest. Many factors are employed to urge students to learn. However, there is no yet absolute agreement on the question whether grades can render students’ encouragement to learn.
As I see it,the answer is by all means!
2理由一 In the first place, grades offer students motives to learn.
3理由二 What is more, grades create a competitive environment of learning.
4理由三 Finally, grades are a good way to measure students’ performance.