how to create captcha control in c#

I need to create a similar captcha as used on this page

http://knowledge.scot.nhs.uk/home/footer/contact-us.aspx

Any ideas how can we create it in c# or download a free one closer to this?
mmalik15Asked:
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anarki_jimbelConnect With a Mentor Commented:
You may not find exactly same example but the example below may help. You will need to generate two numbers instead of one, you don't probably need to convert these numbers to graphics. You need to calculate a result and save it in a session (as it is done in the example) and basically that's it:

http://www.dotnetspider.com/forum/303491-How-add-captcha-challange-registration-webform.aspx
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anarki_jimbelCommented:
You may also search for a solution here at EE by entering a search string like "captcha asp". I'm pretty sure other people asked similar questions before.
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