运行截图如下：
代码如下；
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>CN_TEST1</title><meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no"><!-- bootstrap css核心文件--><link rel="stylesheet" href="bootstrap-4.4.1-dist/css/bootstrap.min.css"><!-- bootstrap使用到了jquery，需要提前引用--><script src="js/jquery-3.5.1.min.js"></script><!-- 弹窗、提示、下拉菜单--><script src="bootstrap-4.4.1-dist/js/bootstrap.bundle.js"></script><!-- bootstrap核心文件--><script src="bootstrap-4.4.1-dist/js/bootstrap.min.js"></script></head><body><div class="container-fluid"><div class="dropdown"><button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">DropDown</button><div class="dropdown-menu"><a class="dropdown-item" href="#">Link 1</a><a class="dropdown-item" href="#">Link 2</a><a class="dropdown-item" href="#">Link 3</a></div></div><br/><div class="dropdown"><a class="btn btn-secondary dropdown-toggle" href="#" role="button" id="dropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown</a><div class="dropdown-menu" aria-labelledby="dropdownMenuLink"><div class="dropdown-header">Dropdown header 1</div><a class="dropdown-item" href="#">Link 1</a><a class="dropdown-item" href="#">Link 2</a><div class="dropdown-divider"></div><a class="dropdown-item disabled" href="#">Link 3</a></div></div><br/></div></body></html>
解释：
①实现下拉菜单的逻辑先在div中设置dropdown-menu，再到里面一层放dropdown-item；
