Read the text below then answer the questions.

Six sentences have been removed from the text. 

Scuba Diving

Scuba diving is a mode of underwater diving where the diver uses an apparatus that is completely independent of surface supply to breathe underwater. The name “scuba”, an acronym of “Self-Contained Underwater Breathing Apparatus”, was first used by Christian J. Lambertsen in a patent submitted in 1952. Sentence A allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers. Although the use of compressed air is common, a mixture of air and oxygen called enriched air or nitrox has become popular due to its benefit of reduced nitrogen intake during long or repetitive dives. Sentence B.

A scuba diver primarily moves underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface. Sentence C, exposure protection by means of a diving suit, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive. Some scuba divers use a snorkel when swimming on the surface. Sentence D by diving instructors affiliated to the diver certification organizations which issue these certifications. These include standard operating procedures for using the equipment and dealing with the general hazards of the underwater environment and emergency procedures for self-help and assistance of a similarly equipped diver experiencing problems. Sentence E but a higher level of fitness may be appropriate for some applications.

The safety of underwater diving depends on four factors: the environment, the equipment, behavior of the individual diver, and the performance of the dive team. The underwater environment can impose severe physical and psychological stress on a diver Sentence F Scuba equipment allows the diver to operate underwater for limited periods, and the reliable function of some of the equipment is critical to even short-term survival. Other equipment allows the diver to operate in relative comfort and efficiency. The performance of the individual diver depends on learned skills, many of which are not intuitive, and the performance of the team Sentence G

Sentence H is in the presence of a similarly equipped person who will understand the problem and can render assistance. Divers are trained to assist in those emergencies specified in the training standards for their certification and are required to demonstrate competence in a set of prescribed buddy assistance skills. The fundamentals of buddy and team safety are centered on diver communication, redundancy of gear and breathing gas by sharing with the buddy, and the added situational perspective of another diver. There is general consensus that the presence of a buddy both willing and competent to assist can reduce the risk of certain classes of accidents, but much less agreement on how often this happens in practice.