下列词语中加线字的字音全都正确的一项是（   ）

设a＜b＜0，a²+b²=4ab，则 $\frac{a+b}{a-b}$ 的值为____．

   The teacher stood before his class and was going to hand out the final exam papers. “I know how  1   you have all worked to prepare for this test,” he said. “And because I know you can do it well, I am willing to offer a B to anyone who would  2  not to take the test. ”
   Many students jumped up to   3   the teacher and left the classroom.  The teacher looked at the   4  students and said, “Does anyone else want to get a B? This is your last   5 . ” Two more students decided to go.     Seven students remained.  The teacher then handed out the  6   .  There were only three sentences typed on the paper: Congratulations! You have just   7   an A in this class.  Keep believing in yourself.     I never had a teacher like that, but I think it is a test  8   any teacher could and should give.  Students who are not   9   in what they have learned are B students at best(充其量). The same is   10   in real life.  The A students are those who believe in what they are doing because they have   11   both successes and failures.  They have learned life’s lessons, either from normal education   12  from events in their lives, and have become   13   people.      Scientists say that by the age of eight, 80% of what we believe about ourselves has already been formed.  You are a big kid now, and you   14   that  you have some limits (限制).  However, there is   15   you can’t do or learn or be.  Sir Edmund Hillary, the first person to reach the summit of Mount Everest said, “It’s not the mountain we conquer(克服) but ourselves. ”