FRANKFURT, Germany (AP) — The European Central Bank left its key interest rate benchmarks and stimulus programs unchanged on Thursday, when it held its first policy meeting under its newly appointed president, Christine Lagarde. The decision follows a similar one by the U.S. Federal Reserve this week to leave rates low as central banks monitor the health of major economies. Investors are now waiting for Lagarde to give her first official assessment of the European economy at a news conference in Frankfurt, Germany. Analysts think Lagarde will stress that the economy still needs support from the central bank, and that policymakers must be on their guard against things turning out worse than expected. Doubts have grown about how much good additional central bank action can do to support developed economies; the U.S.