mysql_query returned false, that caused the error at mysql_num_rows(), you could have concatenated using . in php like this:
WHERE NRC='".$NRC."'");
However, there are two reasons here that you should not do it, rewrite your code instead:
1. mysql extension has been deprecated as of PHP 5.5, it is recommended to use mysqli or PDO, read more:
Choosing MySQL APIs[
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2. Use prepared statement instead of injecting parameters directly in the sql query owing to sql injection risk, read more:
Prepared Statements in PHP and MySQLi[
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