Disabled Submit Button with jQuery Script

agreement.html

<form>
  <input type="checkbox" name="agree" id="agree" value="1">
  <button type="submit" class="btn">Accept Terms & Conditions</button>
</form>

<script>
	$(function(){
		$('.btn').attr('disabled', 'disabled');
		$('#agree').click(function(){
			if($(this).is(':checked')) {
				$('.btn').removeAttr('disabled').addClass('btn-primary');
			} else {
				$('.btn').attr('disabled', 'disabled').removeClass('btn-primary');
			}
		});
	});
</script>

Creating jQuery function that disable submit button when user didn't check the check-box field.