From what hazards must the hose be protected?

The correct answer highlights the importance of protecting the hose from fretting and high heat. Fretting refers to the wear and damage caused by the relative motion between materials, which can lead to deterioration of the hose material over time. High temperatures can also negatively affect the integrity and performance of hoses, especially those used in compressed natural gas applications where heat exposure can compromise the structural integrity and potentially lead to leaks or ruptures.

Protecting hoses from these hazards is critical in maintaining safety and functionality in a CNG system. By ensuring that hoses are shielded from both the mechanical wear of fretting and the damaging effects of high heat, operators can prevent failures that could result in dangerous situations and operational downtime.

The other options, while they represent valid concerns in different contexts, are not the primary focus here. For example, moisture and abrasion can cause issues, but they are secondary to the risks presented by fretting and high heat in terms of immediate operational concerns for CNG systems. Similarly, while dirt and dust can be problematic, they usually don't pose as severe a threat to hose integrity as the combination of heat and mechanical wear does.