The Trump administration on Tuesday advanced a plan meant to expand the U.S. market for corn-based ethanol and place trading restrictions on credits that refiners use to prove they are using biofuel. The Environmental Protection Agency proposal is a first step in fulfilling President Donald Trump’s promise to unleash sales of gasoline containing 15 percent ethanol and deliver at least a symbolic victory to corn farmers in the Midwest who have been battered by his trade fight with China. Air pollution requirements currently block sales of that E15 gasoline from June 1 to Sept. 15 in areas where smog is a problem, and the EPA proposal would effectively lift those restrictions so it could be sold year round.