Предупреждение: mysql_num_rows () ожидает, что параметр 1 будет ресурсом,

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у меня есть ошибка говорит он

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\amargaforum\questions.php on line 39

вот код

Что я уже пробовал:

<?php session_start();
	require("header.php");
	require("checkUser.php");
	
	$id=$_GET["id"];
	$page = $_GET['page'];
    $var=$page*4-4;
?>



<style>
	

.pagination a {
                color: black;
                float: center;
                padding: 8px 16px;
                text-decoration: none;
                transition: background-color .3s;
}

.pagination a.active {
    background-color: black;
    color: white;
}

.pagination a:hover:not(.active) {background-color: #ddd;}
</style>


<a class="btnS" href="question.php?stid=<?php echo $id ?>">Ask Question</a>
<hr />
<?php 
	
	$str="SELECT * FROM question, user WHERE question.user_id=user.user_id and subtopic_id=$id Limit $var,4";
	$result=ExecuteQuery($str);
	
	$no_rows = mysql_num_rows($result);
	
	if ($no_rows > 0)
	{
		while($row = mysql_fetch_array($result))
		{
			$rowsc=ExecuteQuery("SELECT count(*) as counter from answer where question_id=$row[question_id]");
			$rowc = mysql_fetch_array($rowsc);
			$count = $rowc['counter'];
			
			echo "<h4>";
			echo "<span class='box2'>";
			echo "<span class='head'><a href='questionview.php?qid=$row[question_id]' >$row[heading]</a> </span>";
			
			echo "</span>";
			echo "</h4>";
			
			echo "$row[question_detail] <span class='view2'>Views : $row[views], Replies :$count</span>";
			echo "<br/><br/>";
			
			echo "Asked by<br/>$row[fullname]";
		
			echo "<br/><div class='line'></div>";
	
			
		}
	
		
	}
	
			

	else
	{
		echo "No questions at the moment";
	}
	
 

?>

<br><br>
<center> <div class="pagination">
  <a href="">«</a>
<?php
for ($i=1; $i <= 10; $i++) { 
  echo '<a class="active" href="questions.php?page='.$i.'">'.$i.'</a>';
}
?>
  <a href="#">»</a>
</div></center>
<br>


<?php require("footer.php")?>

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