小明使用显微镜时，看不到白亮视野．分析其原因不正确的是（　　）

小萱想利用一块电流表和阻值已知的电阻R₀测量未知电阻R_X的阻值。小萱选择了满足实验要求的实验器材，并连接了部分实验电路，如图所示。

    Do you want to do well in exams? Sometimes your knowledge is not enough， you also need good strategies．Here are some 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 to give one answer， only write one．

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

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

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

一个物体在光滑水平面上以初速度v₀做曲线运动，已知在此过程中只受一个恒力F作用，运动轨迹如图，则由M到N的过程中，速度大小的变化为（   ）

为了除去粗盐中的Ca²⁺、Mg²⁺、SO₄^2﹣等离子，可将粗盐溶于水后进行如下操作：①加入适量盐酸，②加过量NaOH溶液，③加过量Na₂CO₃溶液，④加过量BaCl₂溶液，⑤过滤．下列操作顺序合理的是（　　）