解答 答案：breathing． 根據(jù)提示漢語結(jié)合語境推測句意是"因為空氣稀薄，我們呼吸有很大的困難．"，本題中，由于設(shè)空處單詞放在 had great difficulty后，構(gòu)成句式"have difficulty（in） doing"，所以要填動詞的-ing形式，呼吸的動詞為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．（ 每周五小時 ）
