“一送红军，下了山，秋风细雨，缠绵绵。……问一声亲人，红军啊，几时人马，再回山。”这是电视剧《长征》的主题曲，红军被迫离开中央革命根据地的主要原因是( )
根据对话内容及问题选择正确答案（ ）
The chief engineer of the high-speed railway ________ Shanghai to Beijing always devotes most of his time ________ his work.
