What is the primary purpose of a buoyancy compensator (BC)?

The primary purpose of a buoyancy compensator (BC) is to control ascent and descent rates while providing surface flotation. The BC allows divers to adjust their buoyancy underwater by adding or releasing air from the bladder. This adjustment is crucial for achieving neutral buoyancy, which enables divers to maintain their depth without using energy to swim up or down.

Additionally, when a diver surfaces, the BC provides positive buoyancy, ensuring that the diver stays afloat easily at the surface. This safety feature is essential for recovery after a dive and for resting or waiting for pickup on the surface. Given these roles, both controlling ascent and descent rates and providing surface flotation are integral to the function of a BC, making the combined choice the correct answer.