波动率计算: daily % 1.97, 15.937 (square root of 254), historical vol
http://www.schaeffersresearch.com/streetools/stock_quotes.aspx?Ticker=BIDU
http://www.ivolatility.com/options.j?ticker=bidu&R=0&top_lookup__is__sent=1
http://www.schaeffersresearch.com/streetools/stock_quotes.aspx?Ticker=BIDU
http://www.ivolatility.com/options.j?ticker=bidu&R=0&top_lookup__is__sent=1