So many students -for the traffic jam ,the exam had to be pu

选C,非谓语
主句the exam had to be put off的主谓是完整 的
So many students being late for the traffic jam 用ing表示原因的状语从句,由于主从句的主语不一致,所以从句自己跟上了主语So many students ,如果一致的情况下,从句的主语可以省略!