Questions

The main constituent of biogas is methane (55-70%), CO2 (30-45%) and some traces of gases such as H2S and ammonia.

Biogas is produced under anaerobic conditions; the process is denominated as anaerobic digestion.

The 22nd edition of Asian Athletics Championships (AAC) has started at Kalinga Stadium in Bhubaneswar, Odisha on July 6, 2017 and will continue till July 9, 2017. Athlete Tintu Luka was the flag bearer for the Indian contingent which received a rousing welcome from the crowd. Odia athlete Srabani Nanda had the honour of taking the oath on behalf of athletes at the inaugural ceremony. Over 1,200 athletes and officials from 43 countries in Asia witnessed the largest sporting event. The official mascot of the Championship is “Olly Turtle” that represents the famous Olive Ridley sea turtle.

Water has the maximum density at 4oC. So at this temperature, it sinks down to bottom and stays here. If it cools further, it will become less dense and move up. So while the upper layers can cool enough to form ice, the bottom layers remain as water at  4oC .  

Therefore, (C) is correct.