To be well fit for the job, _____.    [ ]

A. a good knowledge of physics is needed  

B. one needs to have a good knowledge of physics  

C. it is required that one should have a good knowledge of physics  

D. a good knowledge of physics is important

敷设直埋光缆时，根据土质和环境采取适当的加强保护，一般埋深为0.4～0.6m。