The arrows represented by letters A, B, and C reference ________ at the inlet and outlet of the trap seal.

The correct answer highlights the concept of equalized atmospheric pressure at the inlet and outlet of a trap seal. In plumbing systems, particularly when dealing with traps, it is crucial to maintain the balance of pressure at both ends to ensure proper functionality.

The trap seal prevents sewer gases from entering the building while allowing wastewater to flow through. For this seal to work effectively, the atmospheric pressures on both sides must be equalized. This balance helps to avoid the suction that might otherwise break the seal and is pivotal in maintaining the integrity of the drainage system.

Understanding the significance of atmospheric pressure in this context is essential for ensuring efficient plumbing systems. Recognizing how pressure influences the behavior of water and waste flow within traps aids in grasping more complex plumbing principles.