NEW YORK (AP) -- Shares of major U.S. banks plunged Thursday as the government mulled giving Bank of America Corp. a fresh multibillion-dollar aid package, raising fears on Wall Street that the battered financial sector may need an even bigger bailout. Bank of America shares fell as much as 28 percent -- dropping to their lowest level in 18 years -- on news that the bank may seek another capital injection to manage losses from its takeover of Merrill Lynch. Citigroup Inc. shares fell to a near 16-year low as investors braced for horrible fourth-quarter earnings due Friday. And JPMorgan...