The correct option is A
The impressive evolution of bridges that occurs in the second half of the nineteenth century has two fundamental reasons: the steam engine and the railroad. The production of cast iron and wrought iron was linked, at first, to limited local production linked to the humid territories where the existence of forests and large rivers allowed the local production of charcoal for the melting of ferruginous minerals and the use of the force of water to forge iron by beating.
The other reason for the development of bridges is the impossibility of the railway to overcome steep slopes for commercial exploitation, which required the construction of large viaducts throughout the western world to overcome orographic obstacles. The development of rail transport, with higher speeds and load capacity, required the rail bridges to withstand greater overloads and stresses, being the tractions of the construction material which required new materials and new structural calculation.
