In the opinion of Li Ming, a science student in that high school, physics is_______ chemistry, but much easier than English.

A．as difficult a subject as

B．as a difficult subject as

C．a subject so difficult as

D．difficult as a subject as

压力容器的操作人员应持证上岗。压力容器使用单位应对压力容器操作人员进行（）专业培训与安全教育。

A.不定期的

B.定期的

C.自愿的

D.自选的