没有错误.
主动句为：Sb saw the wounded sent to the hospital immediately.
变为被动句为：The wounded were seen sent to the hospital immediately.
主动句采用了“感官动词see + 宾语the wounded + 宾语补足语sent.”,其中the wounded和send之间为被动关系,所以用其过去分词sent.
一楼加上to,其实不对.只有宾语补足语为不带to的不定式（动词原形）时,变被动语态才需要把to补回.
而宾语补足语为现在分词/过去分词时,变为被动句时是不能补to的.