如果您使用视听教具的话，您可以检查一下可用性和可视性。
Check for availability and visibility if you are using audio or visual AIDS.
在以下关键条件下检查系统可用性。
Check the system availability under critical conditions like the following.
本文使用jQuery演示了Ajax用户注册技术，比如，检查用户名可用性，以及当选择的用户名已存在时提示用户名。
The article then demonstrates user registration techniques with Ajax using jQuery, such as checking for user name availability and suggesting user names when a chosen user name is already taken.
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