Where should you put the validation control?

You have a website that includes a form for usemame and password.
You need to ensure that users enter their username and password. The validation must work
in all browsers.
Where should you put the validation control?

A.
in the Web.config file

B.
in the server-side code only

C.
in the client-side code only

D.
in both the client-side code and the server-side code