【正确答案】 1、must have been focused on his book then∥must have been completely wrapped up in his book then    

【答案解析】[解析] 考查情态动词表猜测。对过去某个行为的肯定推测用情态动词must have done。“专注于……”可译为befocused on sth．∥be wrapped up in sth．∥be indulged in sth．。