— What do you think of teaching, Bob?

— I find it fun and challenging. It is a job _____ you are doing something serious but interesting. [ ]

A. where

B. which

C. When

D. that

用于生产和检验的仪器、仪表、量具、衡器等，其（）和（）应符合生产和检验要求，应有明显的合格标志，应定期校验。