A. This burnt off many living forms and it took a long time before oxygen- using life forms started flourishing about 500 million years later.

B. Some of this was ‘fixed’ by iron and organic matter of earth, but the rest soon led the ‘poisonous’ gas, oxygen, attain levels of about 20 per cent in the air.

C. Those days, the earth was rich in a set of microbes called cyanobacteria, which started the early events of photosynthesis, wherein the microbe used CO2 for energy production and emitted oxygen gas as the waste material.

D. One such massive upheaval of the earth’s atmosphere occurred about 2.4 billion years ago, during what is called the “Oxygen Catastrophe”.

E. Cyanobacteria reproduced very fast (doubling every 30 minutes), leading to vast amounts of oxygen in the atmosphere.

A. As people turned to farming, they began to live in fixed settlements, which became small towns.

B. Their labors bore fruit; surplus food freed some of the population from farming.

C. In about 5000 BC, farmers moved down into the fertile river valleys of Mesopotamia, and built dykes and ditches to irrigate the arid land.

D. The cultivation of plants, such as wheat and barley, and the domestication of animals, such as sheep, goats and cattle, began in the Near East in about 8500 BC.

The Indian Space Research Organization (ISRO) crossed an important milestone with the successful launch of weather satellite INSAT-3DR using a Geosynchronous Satellite Launch Vehicle (GSLV) equipped with the indigenous cryogenic upper stage.

A. The September 8 GSLV launch marks the third consecutive success; the fact that it is the first operational flight by the GSLV carrying the indigenous cryogenic upper stage is confirmation that India now belongs to the elite club of countries that have mastered the cryogenic technology.

B. Likewise, igniting a cryogenic fuel and sustaining the combustion for a prolonged period is a daunting task.

C. Maintaining structural and thermal integrity of the engine at very high temperatures during combustion just a few centimeters away from – 250° C, a temperature at which materials behave very differently, is a huge challenge.

D. This marks a departure from the long history of failures with the GSLV; except for the first, every launch of the Polar Satellite Launch Vehicle (PSLV), the workhorse of ISRO, has been a success.

The Thursday launch had fully utilized the maximum payload carrying capacity of the GSLV-Mk II by carrying the heaviest satellite (2,211 kg) ever from Indian soil.

A. Not long after their victory, the Greeks began to think that they would never stack up to the glories of the past.

B. Throughout antiquity, those warriors, especially the Marathonomachai, who fought and defeated the Persian troops in the 490 BCE Battle of Marathon, would be revered as Athens’ Greatest Generation.

C. Even in the mid-fifth century BCE, Athenians were already looking back with longing.

D. A few decades earlier, Athenian citizen-soldiers had helped to rout the Persian invaders of Greece.

A. Interestingly, most creatures, including humans, vocalise using softer organs and membranes that tend to decompose instead of entering into the fossil record.

B. The family of duck-billed dinosaurs known as Parasaurolophus had been at the centre of the paleo-acoustic conversation.

C. They are herbivorous creatures named for the shape of their mouths, but better known for the large, bony crests arching over the back of their heads like a single blunted horn.

D. Parasaurolophus is a rarity in this regard, as no other animal has been known to dedicate so much hard, fossil-friendly tissue to making noise.