What prevents the bottom chords of trusses with long spans from moving as the ceiling finish is applied?

Temporary bracing is the correct choice because it provides the necessary support and stability to the bottom chords of trusses during the construction process. When trusses are installed with long spans, they can be susceptible to lateral movement or distortion, especially as loads from materials like ceiling finishes are applied.

Temporary bracing helps to maintain the correct alignment and structural integrity of the trusses until the permanent bracing or other structural elements, such as ceiling joists, are installed. This support is crucial for ensuring that the trusses do not move out of alignment, which can affect the overall stability of the ceiling system.

The other options, while related to truss and ceiling construction, do not serve the specific short-term purpose of preventing movement during the initial phases of construction. Permanent bracing, for example, is designed for long-term stability once the building is complete, while ceiling joists are part of the system that supports ceilings and can help distribute loads, but they are not utilized during the temporary period of applying finishes. Sheathing refers to material applied to the exterior surfaces of structures and does not play a direct role in maintaining the positioning of truss chords during the initial construction process.