What can you say about an object moving at a constant velocity? *
no forces are acting on it
gravity is exerting the strongest force
the forces acting on it are in balanced
it is stationary

b would be the answer

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 =

answergravity;