题目

    Success in high school is often a stepping stone to a bigger success in life.  36  However,in addition to hard work,there are a number of time-tested ways you can employ to do better in high school.     Find successful friends.Students who make friends with those who do well in school will improve their grades too.  37  In addition,such students can also cooperate by sharing notes and helping each other understand study material.       38  Even if you know the material,it is always better to take notes just in case you may forget something. Additionally,taking notes观察运算过程，其中正确的是（　　）A．1x-12=1x-2B．xx-1-1=1x-1C．tx-t-1x=-1xD．tx+ty=2txy