下列关于阿斯巴甜的说法正确的是（   ）

某兴趣小组同学在做“研究凸透镜成像规律”的实验时，记录并绘制了物体离凸透镜的距离u跟实像到凸透镜的距离v之间的关系如图所示，则凸透镜的焦距为(    )

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    As we all know，having a great memory is important．It can help you succeed in school，in your job，and in life．It also helps you learn faster and better．We all know that fast learner's are always in demand．If you have a great memory, I'm sure you can succeed more easily．

    If you are willing to make a few changes in your life．you should improve your memory first．My plan is to explain every skill that can help you improve your memory．

    You can not only learn memory skills on this website，but also learn how to improve your memory through diet，exercise，sleep and other ways．

    I think that memory improvement can be achieved (实现)in two ways：improving the health of your brain and using memory skills．Many people think they may be difficult．In fact, they are easier to do than you might think，but you have to work hard by yourself.

    Memory improvement needs time and effort on your part．You cannot improve your memory if you do not work on it．Of course，I believe that a more powerful brain is well worth the effort!

    Here are a few of the great memory improvement tips．I will give you some tips in the following part．

下列划线的成语使用有错误的一项是（    ）