从第一个下拉菜单中选择一个选项时，该选项将从列表中删除，并且在第二个下拉列表中将不再可用。
如果从第二个下拉菜单中选择了新元素，则第三个下拉菜单将不再可用。
例如：
Successfully loaded Appfile at path '/Users/xx/Workspace/yy/ci/zz/fastlane/Appfile'
-------
Successfully loaded Appfile at path '/Users/xx/Workspace/yy/ci/zz/fastlane/Appfile'
-------
WARN [2019-05-31 03:15:16.70]: You have required a gem, if this is a third party gem, please use `fastlane_require 'match'` to ensure the gem is installed locally.
INFO [2019-05-31 03:15:17.00]: ------------------------------
INFO [2019-05-31 03:15:17.00]: --- Step: default_platform ---
INFO [2019-05-31 03:15:17.00]: ------------------------------
INFO [2019-05-31 03:15:17.01]: Driving the lane 'mac test'
INFO [2019-05-31 03:15:17.01]: Encrypting git!
INFO [2019-05-31 03:15:17.01]: Cloning remote git repo...
INFO [2019-05-31 03:15:17.01]: $ git clone ./../gitrepo/.git /var/folders/7s/s7248n8j0yz0lth821hx4bdm0000gp/T/d20190531-95641-60lael -b master --single-branch
INFO [2019-05-31 03:15:17.02]: ▸ Cloning into '/var/folders/7s/s7248n8j0yz0lth821hx4bdm0000gp/T/d20190531-95641-60lael'...
INFO [2019-05-31 03:15:17.13]: ▸ done.
INFO [2019-05-31 03:15:17.15]: Successfully decrypted certificates repo
INFO [2019-05-31 03:15:17.15]: Successfully encrypted certificates repo
INFO [2019-05-31 03:15:17.15]: Pushing changes to remote git repo...
INFO [2019-05-31 03:15:17.15]: $ git add /var/folders/7s/s7248n8j0yz0lth821hx4bdm0000gp/T/d20190531-95641-60lael/match_version.txt
INFO [2019-05-31 03:15:17.17]: $ git add /var/folders/7s/s7248n8j0yz0lth821hx4bdm0000gp/T/d20190531-95641-60lael/out.txt
INFO [2019-05-31 03:15:17.19]: $ git commit -m \[fastlane\]\ Updated\ \ and\ platform\
INFO [2019-05-31 03:15:17.24]: ▸ [master 6bdc078] [fastlane] Updated and platform
INFO [2019-05-31 03:15:17.25]: ▸ 1 file changed, 1 insertion(+), 1 deletion(-)
INFO [2019-05-31 03:15:17.25]: $ git push origin master
INFO [2019-05-31 03:15:17.31]: ▸ Enumerating objects: 5, done.
INFO [2019-05-31 03:15:17.31]: ▸ Counting objects: 100% (5/5), done.
INFO [2019-05-31 03:15:17.31]: ▸ Delta compression using up to 8 threads.
INFO [2019-05-31 03:15:17.31]: ▸ Compressing objects: 100% (2/2), done.
INFO [2019-05-31 03:15:17.32]: ▸ Writing objects: 100% (3/3), 289 bytes | 144.00 KiB/s, done.
INFO [2019-05-31 03:15:17.32]: ▸ Total 3 (delta 1), reused 0 (delta 0)
INFO [2019-05-31 03:15:17.37]: ▸ To /Users/xx/Workspace/yy/ci/zz/fastlane/./../gitrepo/.git
INFO [2019-05-31 03:15:17.37]: ▸ cac225c..6bdc078 master -> master
INFO [2019-05-31 03:15:17.37]: Finished uploading files to Git Repo [./../gitrepo/.git]
INFO [2019-05-31 03:15:17.44]: Successfully generated documentation at path '/Users/xx/Workspace/yy/ci/zz/fastlane/README.md'
+------+------------------+-------------+
| fastlane summary |
+------+------------------+-------------+
| Step | Action | Time (in s) |
+------+------------------+-------------+
| 1 | default_platform | 0 |
+------+------------------+-------------+
INFO [2019-05-31 03:15:17.45]: fastlane.tools finished successfully
这是我想出的代码，但是有更干净的方法吗？
HTML：
drop_down1: Dog
cat
mouse -->selected
lion
drop_down2: Dog-->selected
cat
lion
drop_down3: cat
lion
JavaScript：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Ghibli App</title>
<link href="https://fonts.googleapis.com/css?family=Dosis:400,700" rel="stylesheet" />
<link href="style.css" rel="stylesheet" />
</head>
<body>
<div id="root"></div>
<select id="sel1" onchange="show1(this)">
<option value = "">--Select--</option>
</select>
<select id="sel2" onchange="show2(this)" >
<option value = "">--Select--</option>
</select>
<select id="sel3" onchange="show3(this)">
<option value = "">--Select--</option>
</select>
<select id="sel4">
<option value = "">--Select--</option>
</select>
<script src="script.js"></script>
</body>
</html>