The correct option is A: Microorganisms
The largest mass of living material on earth comes from microorganisms.
Microorganisms are tiny microscopic organisms that can not be seen through a bare eye.
In order to see them, we need a high-resolution microscope.
Fungi, bacteria, protozoa, and algae are microscopic organisms that are found both in marine and freshwater.
All these organisms are single-celled and occur in the form of colonies.
The first microbes are expected to be present on planet 'earth' since early times and were discovered back in the 6th century in India.
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