解答 答案：breathing． 根据提示汉语结合语境推测句意是"因为空气稀薄，我们呼吸有很大的困难．"，本题中，由于设空处单词放在 had great difficulty后，构成句式"have difficulty（in） doing"，所以要填动词的-ing形式，呼吸的动词为breathe，故答案是：breathing
17．It is said that thousands of young people may lose their abilities to hear after a few years if they listen to MP4players which too loud for more thanfive hours a week．（ 每周五小时 ）
