What is a key difference between cargo tanks and portable tanks?

The key difference between cargo tanks and portable tanks lies in how they are utilized and filled. Cargo tanks are specifically designed to be filled while they are on the vehicle, as they are an integral part of the transport system. They are constructed to handle a variety of hazardous materials during transport on a vehicle or vessel.

On the other hand, portable tanks have the flexibility to be filled either while on the vehicle or removed from it, allowing for more versatility in handling and storage. This characteristic of portable tanks is significant because it implies they can be used in different scenarios, including transport, storage at a facility, or when connected to equipment for dispensing or processing materials.

The other statements don’t accurately reflect the operational capabilities and definitions of cargo and portable tanks. For example, saying cargo tanks are always larger than portable tanks is misleading because size can vary based on specific designs and purposes. Similarly, the claim that portable tanks are exclusively for solid materials is false, as they are utilized for liquids and gases as well. The restriction on filling portable tanks only off the vehicle does not apply, which further highlights why option C is the correct distinction.