“它排除了丞相个人专制，相权过大威胁皇权而出现的政治危机，而且增强了决策施政的程序性和合理性，提高了行政效率。”材料中的“它”是指

A．西汉的郡国并行制

B．唐朝的三省六部制

C．元朝的行省制

D．清朝的军机处

–Have you studied English for a long time?

--Yes, I have studied it ______.

A．five years before

B．five years ago

C．for five years

D．since five years