以下是一段CSS代码，可以将一个`<a class="btn">`标签转换成一个美观的按钮：
```
.btn {
  display: inline-block;
  padding: 0.5rem 1rem;
  font-size: 1rem;
  font-weight: bold;
  text-align: center;
  text-decoration: none;
  color: #fff;
  background-color: #007bff;
  border-radius: 0.25rem;
  transition: background-color 0.3s ease;
}

.btn:hover {
  background-color: #0069d9;
}
```
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