I know the defaultchecked is when i enter the page, the radiobutton is checked by default.
but the radio button can't let the user choose either of them.
when the user click on 1st radio button, then checked this button, and set the 2nd radio button checked to false.
if 2nd button is checked, then set the first button checked to false.
only one of them checked.
but now when i code rad1.defaultchecked=true, it is always true, even i click another button, it can't change the checked value to the button clicked.