Measures have been taken to prevent the accidents, but much _____ for more improvement.

A．remains to be done

B．is remained to do

C．remains to do

D．is remaining to be done

城市规划法规体系按其构成特点可分为( )两大类。

A．纵向体系和横向体系

B．水平体系和垂直体系

C．主干法和辅助法

D．主干法和次干法