Which of the following organisms is used as an indicator of water quality?

Who was the first discoverer of antibiotics?

Where are microbes found?

$A$: Microbes have been used for the welfare of humanity.
$R$: The fields of biotechnology and genetic engineering have developed.

The chemical substances which are produced by some microbes and can kill or retard the growth of other microbes are known as:

From the given below,which one increases vitamin $B_{12}$ in our stomach and also plays a very beneficial role in checking disease-causing microbes?