Stocks closed with a flourish Thursday, although volume dwindled on both exchanges, according to preliminary data. The Nasdaq and S&P 500 rose 1% and the Dow Jones industrial average gained 1.2% in the stock market today. Generic drugmakers were the top-performing group, up more than 7%. Dow component Pfizer (PFE) agreed to buy Hospira (HSP), a member of the 17-member group, for a 39% premium over Wednesday’s close.