how to apply css to a link within a particular div only?

Here is my situation. I have 3 divs on my website. There is a button in each of divs. Here is my codes :

<div id="gg1">
  <a class="btn">
</div>
<div id="gg2">
  <a class="btn">
</div>
<div id="gg3">
  <a class="btn">
<开发者_StackOverflow社区/div>

May I know how to apply css style to 1 button only without effect on other 2 buttons Css or Jquery? Note that I cannot change the class and id of the buttons.

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Use a descendant selector:

#gg1 a.btn {
    /* styles here affect only a.btn elements within #gg1 */
}