They walked in the streets for two hours carrying a sign whi

主语是they,walk是谓语,in the streets 是地点状语,for tow hours 是时间状语,carrying a sign 是伴随状语,表示边走边说,walk的时候,举标志这个动作正在发生,所以用carrying。which said,"we need two tickets。"做 sign的定语从句,表示sign上写的内容。不能用what,因为这句“we need two tickets”,是sign上面写的,在定语从句里,要用which,或者that来引导。而关系代词“what”一般引导的是宾语从句,不能引导定语从句。