Hi teachers,<br/><br/>This is excerpt in the news:<br/><br/>The sources of melamine contamination <b><i>has</i></b> been narrowed it down to 2 production lines on the mainland.<br/><br/>Can you tell me why "has" is used and not "have"? "The sources" should take "have", not "has"?<br/><br/>Thank you.<br/><br/>Tinanam