Workers who had lost their jobs left the factory _____ very bitterly about the way they had been treated

此题考察的是英语中非谓语动词的使用。根据句子结构，"left the factory" 是主句，"_____ very bitterly about the way they had been treated" 是伴随状语。伴随状语通常由现在分词或过去分词来引导，表示伴随动作。由于主语 "Workers" 和 "complain" 之间是主动关系，所以应该使用现在分词 "complaining"。