Read the following passage carefully and answer the questions that follows:

Environmental factors may create conditions that are either negative or positive for various life forms. Free oxygen supercharged respiration and made complex life possible as atmospheric O2 became increasingly available between 2.2 billion years (by) and 55 million years (my) ago. The O2 approached todays levels around 550 my, when complex life began to get larger. The O2 levels reached their highest (>1/3rd of all atmospheric gases) for all time on earth around 300 my. Bugs became bigger i.e. land arthropods were less-limited in size. Massive millipedes grew to a length of over three meters and half a meter wide. This could not happen today, it is obviously genetically possible, but it is not environmentally-possible. The bodies of spiders were as large as man's head; today's O2 levels do not permit such a physiological possibility. Cockroaches were everywhere, and some were really large (> 10 inches long and 5 inches wide). Predatory dragonflies, flying up into new highly -oxygenated aerial environment, had wingspans exceeding two meters. The First reptiles and the amniotes egg also appeared at this time; the moisture-proof skin and egg allowed these vertebrates to truly become land animals. The warm and moist tropical environment of the Carboniferous Period (354-290 my) with the high oxygen levels (nearly 40% higher than today), gave way to a cooler and drier Permian Period (290-250 my); reptiles continued to flourish. A slight drop in atmospheric oxygen changed the conditions of life for the large land arthropods. Life goes on only if the specific conditions of life can be found. Even though there was no mass extinction at the Carboniferous-Permian transition, the high-atmospheric oxygen-dependent large land arthropods of the Carboniferous Period were never to be seen again. Smaller arthropods continue to exist in the lower atmospheric oxygen conditions.