单项选择题

A.层高不大于5m时为6mm,层高大于5m时为8mm
B.层高不大于5m时为8mm,层高大于5m时为6mm
C.层高不大于5m时为7mm,层高大于5m时为5mm
D.层高不大于5m时为5mm,层高大于5m时为7mm