学进去

阅读短文，根据短文内容回答问题．
When it comes to the letter grade on your test or homework, you might notice that there is no letter E. Have you ever thought about why that is so?
In the A, B, C, D and F grading system, the first four letters are typically considered passing grades. An F in this system simply stands for“fail”.The word“fail”happens to start with the letter F, which seems to leave out the letter E. The fact”．Some schools have U grade for“unsatisfactory”,or I grade for “incomplete”.

Even with all this said, we should also point out that E grade actually has been
used pretty commonly throughout the history of letter grades in the US.
The first college in the US to use a letter grading system like the ones we use today is Mount Holyoke College. In 1897,they began to use the following grading scale:
A: 95-100%(excellent)       B: 85-94%(good)      C: 76-84%(fair)
D: 75%(barely passed)       E: below 75%(failed)
We can see the inclusion of E instead of F.
Gradually,the letter grading system became more popular throughout the US. However, many schools decided to drop the E grade and go straight to F.There is no evidence(证据) to really support this,but one possible explanation is that teachers were worried that some students and parents might mistake E for “excellent”.
Why do we have letter grades? Well,part of the reason is that they made grading simpler during a time of great change for schools. As the 20th century began,growing cities and an increase in immigration led to larger school classrooms. Most teachers at that time thought this new letter grading system was an easy,fair and clear way to grade students.
Today,more and more people argue that letter grades don’t fully reflect(反映) student learning。However，as teachers try to improve grading methods,many parents continue to favor the letter grades they got as kids, they are familiar(熟悉) and easy for parents to understand. So while they might not be perfect, the letter grades probably aren’t going away any time soon.
1．what does an F stand for in the A、B、C、D and F grading system?
2．when was the letter grading system first used in the US college?
3．what is one possible explanation for dropping the E grade?
4．what did most teachers think of the letter grading system in the 20th century?
5．why do many parents still prefer the letter grades today?