What was first responsible for changing the formerly poisonous atmosphere to an oxygen-rich atmosphere as we have today?
meteor impacts
volcanic gases
earthquakes
photosynethic bacteria (cyanobacteria)

The correct answer is cyanobacteria (photosynthetic bacteria)

The majority of the scientists considers that for about half of the history of the Earth, the atmosphere comprises no oxygen. It was blue-green algae or cyanobacteria, which became the first thing to generate oxygen by the process of photosynthesis, that is, about 2.7 billion years ago.  

Cyanobacteria are the microbes, which thrives mainly in the waters of the sea. They are considered to have been the initial species on Earth to conduct oxygenic photosynthesis. In the process, they generated organic carbon, that is, the building materials of life's molecules, and discharged oxygen gas in the process. The oxygen entered into the seawater, and from there some of it got escaped out into the atmosphere.  

there are no statements provided but here is why:

sulfur belongs to group 16 because it can be found in nature whether it is in a free or combined state.

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1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%