U.S. stocks fell sharply in heavy trading, yesterday, and the S&P 500 and the Dow had their worst day since October, after a collapse in Greek bailout talks intensified fears that the country could be the first to exit the euro zone. The European Central Bank froze funding to Greek banks, forcing Athens to shut banks for a week to keep them from collapsing. Greece appeared to confirm it was heading for a default after a government official said the country would not pay a 1.6 billion euro loan installment due to the IMF tomorrow. U.S. investors also worried about Puerto Rico's debt problems, and a bear market in China. So the Dow Jones industrial average plummeted 350 points, or 2 percent, to 17,596. The S&P 500 tumbled 44 points, or 2.1 percent, to 2,058. And the Nasdaq Composite sank 122 points, or 2.4 percent, to 4,958. The CBOE Volatility index, a measure of the premium traders are willing to pay for protection against a drop in the S&P 500, jumped 34.5 percent, to its highest level in almost five months.