單項(xiàng)選擇題鋼筋籠的保護(hù)層厚度以設(shè)計(jì)為準(zhǔn),設(shè)計(jì)沒作規(guī)定時(shí),可定為()

A.30~50mm
B.50~70mm
C.70~90mm
D.90~110mm