If user click one of radio button, how can it makes text box from dim to un-dim

 Hi,
I am doing payment test page, and have following tools:
RadioButtonList - choose cash or credit card

TextBox - store credit card no

Button - submit

if user click RadioButtonList e.g. choose  "credit card" item, then the credit card textbox is changed to un-dim or textbox readonly = false or something like that,  how can I find this code. Thanks.
 
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