WASHINGTON (AP) — An index that tracks the number of signed contracts to buy U.S. homes rose to its highest level in nearly two years last month, the latest sign the battered housing market is slowly improving. The National Association of Realtors said Thursday that its index of sales agreements increased 4.1 percent last month to a reading of 101.4. That's the highest since April 2010, when buyers could qualify for a federal home-buying tax credit. A reading of 100 is considered healthy.