What are the two critical factors to consider when installing a shower drain?

When installing a shower drain, the size of the horizontal line and the distance from the fixture's trap to the vent are indeed critical factors. The size of the horizontal line is essential because it must be adequate to handle the flow of wastewater from the shower to prevent clogs and ensure proper drainage. If the pipe is too small, it may result in slow drainage and increased likelihood of backups.

The distance from the fixture's trap to the vent is also crucial, as proper venting is necessary to allow air to enter the drainage system. This air intake prevents vacuum formation when water is draining, which could lead to slow drainage or siphonage of water from the trap, allowing sewer gases to enter the living space. Proper venting helps maintain the function and safety of the plumbing system.

In contrast, the other factors mentioned in the alternatives don’t address the core requirements for proper drainage. For instance, the size of the shower drain cover and its location might influence aesthetics or convenience but do not affect the plumbing functionality as directly as the horizontal line size and vent distance. Similarly, while the type of pipe and drum trap relate more to specific installation methods, they are not as central to the drain's immediate functionality as the considerations chosen in the correct answer.