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.  

A

Explanation:

Elements in the same group of the periodic table have similar chemical properties. Elements can be classified as metals, metalloids, and nonmetals, or as a main-group elements, transition metals, and inner transition metals. Groups are numbered 1–18 from left to right.

P.S, All elements in a row have the same number of electron shells. Each next element in a period has one more proton and is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law.