css如何选择相同类名下的第一个元素

Question

 <h1 class='c'>This is a heading</h1>
 <p class='b'>The first paragraph.</p>
 <p class='aa'>The second paragraph.</p>
 <p class='aa'>The third paragraph.</p>
 <p class='aa'>The fourth paragraph.</p>

我想选择类名为aa的第一个元素，怎么选择？ p.aa:nth-of-type(1) 好像不管用

Answer 1

<script>
       document.getElementsByClassName('aa')[0].style.color="blue";
   </script>