The river -------- (become) dirty since the paper factory wa

正确答案：填has become
即：The river ( has become ) dirty since the paper factory was opened up.
翻译：自从这个造纸厂开业以来,这条河流已经变脏了（被污染了）.
解析：since 是现在完成时的标志.
有不明白的请继续追问,