单项选择题

A.当梁高≤350时为6mm,梁高>350mm时为8mm
B.当梁高≤450时为6mm,梁高>450mm时为8mm
C.当梁宽≤350时为6mm,梁宽>350mm时为8mm
D.当梁宽≤450时为6mm,梁宽>450mm时为8mm