Javascript Q&A Quiz not working

here is my code. I want to make a javascript based quiz for mcqs. There is a problem while making the verify() function. How to select the user selected option and then match it with the correct answer of that question and tell the status? Thanks

function verify() {
  var correct = 1;
  var selected = 0;
  if (getElementByID.r1) selected = 1;

  if (selected == correct) window.alert("You are right");
  else window.alert("You WRong!!!");

}

<img src="//placekitten.com/g/50/50" alt="x" />
<br><br><br>
<input type="radio" id=r1 /><strong>1. </strong>
<p id=ans1>This is cloud</p>
<br />
<input type="radio" id=r2 /><strong>2. </strong>
<p id=ans2>This is hat</p>
<br />
<input type="radio" id=r3 /><strong>3. </strong>
<p id=ans3>This is rough</p>
<br />
<input type="radio" id=r4 /><strong>4. </strong>
<p id=ans4>This is none</p>
<br />

<input type="button" onclick=verify(); value="send" />
<input type="button" onclick=reveal(); value="Reveal Solution" />