Political figures and the wealthy tend to receive the greater part of the media and public's attention, and there is nothing wrong with that inherently. But when we hear "America singing", the voices of the wealthy and political figures will naturally get lost amongst the sea of voices that are your average Americans. Your working class Americans make up a huge part of the national population. Walt Whitman wanted to express his pride and support for the nameless Americans who are the real backbone of our society. The boatmen, the mechanics, the carpenters and masons all serve a vital function in our society.

                                                        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.

i think the answer is d.