how to create dependent drop down list

[Ashwin]

Exactly what I need means, for example, A drop-down list  is available b drop-down list  Should in  hidden, If the person select A drop-down list  afterward only it need to show B drop down List,

[Joe Pitt]

JIRA has a cascading select list but the second list is always available with values based on the selection of the first list, If that behavior doesn't work for you there aren't any out of the box solution for what you're asking. I'm not sure what plugins are available in the cloud, but some of them do support displaying a field if another is selected.  You may also be able to do it with scripting.