Please explain the 1st question here. I chose A.

The passage is primarily concerned with
(E) the fabrication and construction methods of the box girder bridge.

Explanation:
- The passage opens by defining the box girder bridge (evolution of plate girder, made from concrete or steel).
- The bulk of the passage is dedicated to the two main construction methods:
1. Concrete: "typically cast in place (using falsework supports...)"
2. Steel-plate: "may be preassembled at a fabrication yard and then placed using cranes."
- The passage is a direct description of the material used for fabrication and the methods used for construction.
- (A) is too specific (only steel, ignores concrete).
- (B) is too specific (only focuses on the limited access scenario, which is just one detail about the steel method).
- (C) is too specific (only mentions trestles briefly as an example).
- (D) is too specific (only mentions the plate girder as an introductory historical detail).

According to the author, the girder bridge construction method employed where access for construction is limited to periods of light traffic is the
(C) preassembled method.

Explanation:
The passage links limited access to the steel-plate method:
"...the steel-plate type may be preassembled at a fabrication yard and then placed using cranes. The latter method is often used in situations where access for construction is limited due to traffic..."
- The "latter method" refers back to the one that uses preassembly.
- (E) is partially correct, as the method is used for steel-plate bridges, but the key distinction of the method itself is that the pieces are preassembled (C) off-site, allowing for quick placement when traffic is detoured.