设N_A为阿伏加德罗常数的数值，下列说法正确的是（   ）

根据短文内容，选择正确答案。

Dear Ken,

      I'm glad to hear from you. You asked me how to do well in English exams. Sometimes your knowledge is enough, but you also need good strategies.

 Here are some advice for you:

      Before you answer the questions, if you feel a little nervous, take a few deep breaths to help you relax.

      Always read the question carefully and make sure that you understand it. Read it a few times if possible.

      Find out how much each question is worth or how many points you need.

      If it asks you to give one answer, only give one.

      If there is a difficult question, don't worry. Go on with the next question and come back to it if you have time later.

      Try not to leave questions unanswered when you finish the text. Sometimes a guess may help you get points.

      When you finish, go back and check your answers. You will need to check your writing or spelling mistakes.

     Best wishes to you and your family.

                                                                                                                                                                                Yours,

                                                                                                                                                                                   Amy

下列运算正确的个数为
①（-10）-（-10）=0；②（-3）-（+7）=-10；③0-7=7；④$\frac{5}{6}+\left(-\frac{1}{6}\right)=\frac{2}{3}$

分式$\frac{1}{a^2-2a+1}$，$\frac{1}{a-1}$，$\frac{1}{a^2+2a+1}$的最简公分母是（　　）