For dives with an in-water decompression time greater than 120 minutes, what must be used?

Using a bell for dives with an in-water decompression time greater than 120 minutes is essential because it provides a controlled environment for divers during the decompression phase. This equipment allows for a safe and stable decompression process, reducing the risks associated with long-duration dives.

Diving bells are typically pressurized and can accommodate divers while they conduct decompression in a controlled atmosphere. This is critical for managing the physiological stresses on the body that come from ascending from deeper depths. The use of a bell also enhances safety by being equipped for emergencies, ensuring that divers have a safe space to remain while they decompress instead of relying solely on ascent techniques.

The other options are not suitable for extended decompression. A standard air tank may not provide the necessary support for long dives, as it restricts the duration of air supply and doesn't assist with the complexities of prolonged decompression. An oxygen mask can be used in certain scenarios, but it is not designed for such extended decompression needs. A dive raft lacks the structural integrity and life-support systems required for safe decompression at significant depths and timeframes.