Two identical freight cars roll toward one another, on a level track with negligible friction. One car is moving at 2 m/s and the other car at 1 m/s. After the collision, both cars are coupled and roll together with what speed?

b

explanation:

most of them using statistics and defining how things are being measured

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 =