unable to select one radio button at a time

unable to select one radio button at a time

I am unable to select one radio button at a time. Multiple buttons are
getting selected. I am newbie to html. This is my code. Please help.
<form name="ans_a" onsubmit="return answer_a()">
<table align="center" border="1">
<br>
<br>
<tr>
<td width="500px"> ABC
<br>
<input type="radio" name="A" value="a" id="radio1"> A &nbsp Option A <br>
<input type="radio" name="B" value="b" id="radio2"> B &nbsp Option B <br>
<input type="radio" name="C" value="c" id="radio3"> C &nbsp Option C <br>
<input type="radio" name="D" value="d" id="radio4"> D &nbsp Option D <br>
<br>
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp &nbsp
<input type="button" name="ans1" value="Next" onclick="answer_a()">
</td>
</tr>
</table>
</form>