<qa>  What is the primary study hypothesis for pembrolizumab?This is a study of pembrolizumab MK-3475 for advanced gastric or gastroesophageal junction adenocarcinoma; pembrolizumab will be given as monotherapy to participants who have had previous treatment or who are treatment-naïve; pembrolizumab will also be evaluated as combination therapy with cisplatin and 5-Fluorouracil 5-FU or Japan only capecitabine in treatment-naïve participants. The primary study hypothesis is that pembrolizumab will provide a clinically meaningful Overall Response Rate ORR. 