The best indicator of the efficiency of exercise is

A good indicator of efficient exercise is a good resting heart rate.  This enables athletes and health buffs to meet the needs of their body with a slow heart rate.  It does not take them much effort in terms of physical activity whereas those who are out of shape would exert a lot of effort to complete a task.

                                                        water

a colorless, transparent, odorless, tasteless liquid that forms the seas, lakes, rivers, and rain and is the basis of the fluids of living organisms.

                                                        halon  

any of a number of unreactive gaseous compounds of carbon with bromine and other halogens, used in fire extinguishers, but now known to damage the ozone layer.

                                                  dry chemical

extinguishing agent used on class a, class b, and class c fires.

                                                  carbon dioxide

a colorless, odorless gas produced by burning carbon and organic compounds and by respiration. it is naturally present in air (about 0.03 percent) and is absorbed by plants in photosynthesis.