Innovative approaches to manufacturing, coupled with the tremendous size of the domestic market, led to the emergence of the United States as an industrial giant.

A．followed by

B．deriving from

C．combined with

D．mixed with

现浇钢筋混泥土楼梯有（）两种结构形式。

A、桥式

B、梁式

C、板式

D、双跑式