in thought completely, he didn’t even know what was going on. A. LosingB. LostC. Having l

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A. Losing
(答案→)B. Lost
C. Having lost
D. Having been lost
解析：句意强调主语沉思，而不知道下一步将要做什么事情。由主语he，我们可知应用主动语态，排除选项D；Having lost表示动作发生在主语动作之前，不和题意，排除C；过去分词作形容词，可以表示主语所处的一种状态，符合题意。