RAILROADS?

15)WHAT ARE THE MOST COMMON MATERIALS USED IN BRIDGE CONSTRUCTION?

16)WHAT WAS THE PROBLEM WITH IRON BRIDGES?

17)NAME AND LOCATE THE FIRST STEEL BRIDGE IN THE WORLD.

18)WHAT CAUSED THE TACOMA NARROWS BRIDGE TO COLLAPSE?

19/20) NAME AND LOCATE 4 FAMOUS BRIDGES IN THE UNITED STATES.