Within the Red Hat certification program, the RHCA is considered the most complete (and grueling) certification among the seven major Red Hat certifications. In fact, some equate the prestige of the RHCA to that of the Microsoft Certified Master and possibly the CCIE.
A plumbing fixture like a sink can not function properly without a sink vent. When a drain pipe drains it creates a negative pressure in the sink drain; the negative pressure will pull the water out of the p-trap of the sink that's draining and sewer gases can enter your home if a sink vent is not installed.
A return air grille is often fitted with a filter to trap smaller particulate materials before they have a chance to get into the ductwork. This limits the amount of cleaning needed and also keeps the system running smoothly, reducing the risk of clogs caused by dust, pet hair, and other materials.