A Study of HX008 Compared to Chemotherapy in the First-Line Treatment of Subjects With MSI-H/dMMR Metastatic Colorectal Cancer

Drug: HX008 200 mg ,Q3W

"Drug: bevacizumab 5mg/kg given by IV every 14 days. Other Name: Avastin~Drug: cetuximab 400 mg/sqm initial dose, then 250 mg/sqm once weekly thereafter. Other Name: Erbitux~Drug: oxaliplatin 8 5 mg/sqm by IV, day1. Component of mFOLFOX6.~Drug: irinotecan 180 mg/sqm by IV, day1.Component of FOLFORI.~Drug: calcium Folinate 400 mg/sqm by IV, day1.Component of mFOLFOX6 or FOLFORI.~Drug: 5-fluorouracil 400 mg/sqm, day1,followed by 2400 mg/sqm iv infusion over 46\~48 h.Component of mFOLFOX6 or FOLFORI."