which is different from the annual flu vaccine 为非限制财么性定语从句，修饰 Th序e new vaccine
故正确答案为[B]。
【句子结露模张于构分析】
The new vaccine, ①which is different from the annual flu vaccine, is available ②ahead of expectations.
帝本句为主从复合句，主句主干 T既历里沉he new vaccine...is available 为主系表结构。
①为 which 引导的非限制性定语从句，修渐行叶把物使饰 The new vac弱cine；②为介词结构作时间状语，修饰主句。
