单项选择题
A.25mm B.28mm C.30mm D.32mm
(A)Ⅲ、Ⅳ (B)Ⅰ、Ⅲ (C)Ⅰ、Ⅱ、Ⅲ (D)Ⅰ、Ⅱ、Ⅲ、Ⅳ
A.无厚度规定 B.厚度规定与板跨度有关 C.60mm D.30mm