Radioactive tracers are useful in
question 8 options:

calculating the ages of fossils

determining medical problems

fueling nuclear reactions

treating brain tumors

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 =

the doppler effect is used by weather forecasters to measure 

moving clouds