How to set the TEXTBOX is read only by clicking the radiobutton

 I have to two Radio Buttons in a group.(eg: name of the Radio buttons is YES and NO and the Group name is YN) and one TextBox.

In My Requirement: If I click YES radio button, the textbox wil be read only otherwise it will not  ....but i dono know how to write the code to get the radio button values,checked the condition and based on the condition (if YES) the text box wil be read only..........
 Thanks in Advance..............

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