The students ______ busily when Miss Brown went to get a book she ______ in the office.[ ]

A. had written; left

B. were writing; has left

C. had written; had left

D. were writing; had left

加氢裂化原料要求控制最为关键的指标为（），它不仅影响催化剂的稳定性，还影响催化剂的活性。

A.金属Fe离子含量

B.残炭含量

C.硫含量

D.干点和氮含量