李白在长江上，写下了两首情绪不同的诗，有“平身不下泪，于此泣无穷。”也有“两岸猿声啼不住，轻舟已过万重山”。李白的情绪变化主要是因为（    ）

你认为减少酸雨的途径可以采取的措施是(   )

①少用煤作燃料 ②把工厂烟囱造高 ③燃煤脱硫     ④在已酸化的土壤中加石灰 ⑤开发新能源

    We live on the Earth．The Earth was ____ ，but now some parts are dirty．Let's work together to keep the Earth pretty and healthy because the Earth is only home for all the living things．We should ____．We should ____．We also need water and trees．So we shouldn't  ____．We should stop  ____．

血液通过肺循环后，血液成分的变化（　　）