My application is in mvc 3

I have a ddlfor in my view which lists two values A and B.
There is one more field email in same view
I need to make this email field a required or mandatory filed only if the selected value of the ddl is 'A'.And no need to validate if the value selected is 'B'.
How can i implement this scenario.PLsss help me

Now in my model, email is a mandatory field. Do i need to change that?
If i change that where i will make email mandatory and not mandatory based on ddl value?

Pls advice me.
Thanks in advance,
Priya