Why are only specific colors absorbed and emitted?

The electrons in an atom can only occupy certain allowed energy levels. ... Only certain energy levels are allowed, so only certain transitions are possible and hence specific wavelengths are emitted when an electron drops to a lower energy level.

Explanation:

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =