下列句子中划线成语使用正确的一项是（    ）

      If you want to be a better reader，what should you know about the speed(速度)of reading? Some people read very quickly，when others read very slowly. But which one is better? The quick reader may be a good reader when he reads a story-book for fun. But he may not be able to slow down to read instructions(说明书) carefully. The slow reader may be a good reader. But he may spend too much time reading a story for fun. So the reader who reads everything quickly or the one who reads everything slowly isn't really a good reader. Here are four things to remember about the speed of reading.
       Knowing why you are reading will help you to know whether(是否) to read quickly or slowly.
Some things should be read slowly throughout. Examples(例子)are instructions for making or doing something. Math books，science and story-books are full of important information. You must read these things slowly to understand and remember every important idea.
     Some things should be read quickly throughout. Examples are stories for enjoyment，letters from friends，and some news from newspapers.
In some of your reading，you must change your speed from fast to slow and slow to fast. You will need to read some pages quickly and then slow down and do more careful reading when you come to important ideas.

— No, you ________, my dear. You're free to make your own decision.

已知等腰三角形的两条边长分别是2和4，则它的周长是（    ）