He was sitting there,------- in deep thougut.

在句中：lost是lose的过去分词.
He was sitting there,_______ in deep thought.（他当时正坐在那里,迷失在深思当中）
句中用lost过去分词,表示被动语态.
若用现在分词losing,表示主动语态,他主动的迷失在深思当中,与句意显然不符合.
losing是lose的现在分词.lost还可作为形容词,但在该句子中不是形容词,而是过去分词.
另外,thougut输入错误,应该是thought.