Accounting questions continued to unnerve investors Tuesday, pushing shares sharply lower in light volumes. Major averages ended at their lows. The S&P 500 fell 20.66, or 1.9 percent, to 1083.52, while the Nasdaq Composite dropped 54.59, or 3.0 percent, to 1750.61. The Dow Industrials lost 157.90 to 9745.14. Despite the heavy losses, volumes were among the lightest of the year. Dealers say investors are sidelining cash, waiting for accounting questions to quiet and the economic pace to quicken. //MarketNews