selection in one ddl field makes another ddl as a required field.

I have to accomplish two scenarios:

1. I have two dropdownlist fields say A and B

and A has options as 'xx','aa','bb','yy','ss'

A is a required field, if user selects 'xx' or 'yy' from the dropdownlist A then I need to make B as a required field.

 2. There is a datefield which is not required field. I need to let user enter 'N/A' in this field if they do not have any relevant date , the way i was trying to accomplish this task is :

provide a checkbox in front of this date field, if user checks it  I will fill it with 'N/A'

 

Please provide any direction,link or suggestion how to accomplish the above two scenarios.

Thanks.
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