---Could you explain to me what an adventure is, Mr. Smith?

---An adventure doesn't necessarily mean doing something rough and dangerous. It may be _____ that you have never experienced before. [ ]

A. everything

B. something

C. anything

D. nothing

某医生接待了因手被割伤前来就医的甲、乙两位病人，在详细询问后，决定给接过破伤风疫苗的甲注射破伤风类毒素；给未接种过破伤风疫苗的乙注射抗破伤风抗体。请回答下列问题：

甲注射的破伤风类毒素是破伤风外毒素经甲醛作用4周后脱毒形成的，从免疫学上说类毒素属于（）。