get down to 的TO是介词还是一般用法?

问题描述:

get down to 的TO是介词还是一般用法?
After the rain,you should get down the room.
A.to repair B repair C repairing D to repairing
选择D 为什么词典里只有不定式而这里却是介词?

get down to doing
固定短语 ,to 为介词,后跟名词或者动词ing
类似的还有
look forward to doing等等