如何在从下拉菜单中选择特定选项时更改段落？

<select id="dropdown">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<p id="paragraph">这是一个段落。</p>

document.getElementById("dropdown").onchange = function() {
  // 在这里编写代码
};

document.getElementById("dropdown").onchange = function() {
  var selectedOption = this.value;
  
  var paragraph = document.getElementById("paragraph");
  
  if (selectedOption === "option1") {
    paragraph.innerHTML = "选项1被选中，段落内容已更改。";
  } else if (selectedOption === "option2") {
    paragraph.innerHTML = "选项2被选中，段落内容已更改。";
  } else if (selectedOption === "option3") {
    paragraph.innerHTML = "选项3被选中，段落内容已更改。";
  }
};