Air Canada Stock Plunges on Review  MONTREAL (Reuters) - Shares of Air Canada  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AC.TO target=/stocks/quickinfo/fullquote"&gt;AC.TO&lt;/A&gt; fell by  more than half on Wednesday, after the Toronto Stock Exchange  said it was reviewing the company's stock to determine if it  meets listing requirements.