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Suddenly, a light ____ ____. I saw a man in black standing in front of me.
蒸熟的馒头松软多孔,是因为和面时加入了酵母菌,使面粉发酵产生的____气体遇热膨胀的缘故.
如图,如果∠1+∠2=180°,那么( )