Lackluster Bank of America profits and an apparent drop in demand for iPods and iPhones dragged stocks lower, as did new signs of weakness in Europe, where car sales are plunging and unemployment is rising, and China’s report on Monday that its growth was slower than expected. Producers of metals, energy, and other commodities have been hit hard this week. The Dow fell 138 Wednesday, wiping out most of Tuesday’s gains. The Dow, at a record high last Thursday, is down 1.7% this week; the S&P 500 is off 2.2%. Stocks surged in the first quarter but weak hiring, retail sales, and profits since then suggest growth is hardly robust.