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 “Up until about five years ago, students at this school could have worn anything they wanted on Halloween,” said Rosemarie Nielson, a sixth-grade teacher at St. Theresa School, a Roman Catholic elementary school in the Pelham Bay section of the Bronx where there is a prohibition on toy weapons. “When you consider all the horrific things that have happened in recent years, including 9/11, I can’t blame any school for wanting to stay away from anything that might promote violence,” Ms. Nielson said. Mary Ellen Manniello, whose daughter, Courtney, 9, is a fourth grader at St. 在△ABC中，∠A=50°，∠C=60°，则∠B=（    ）。