A Multi-center, Randomized, Double-blind, Phase II/III Clinical Trial of AK104 or Placebo in Combination With Chemotherapy as Second-line or More Lines for Recurrent or Metastatic Nasopharyngeal Carcinoma (NPC)

To compare whether AK104 combined with GP or PFLL can improve survival benefit, safety and tolerability in nasopharyngeal carcinoma patients who have failed first-line treatment; To compare the survival benefits of GP or GFLL treatment in nasopharyngeal carcinoma patients who had failed first-line therapy; To compare the survival benefits of GP or GFLL combined with AK104 in patients with nasopharyngeal carcinoma who had failed first-line therapy.