2. Timers at a track meet used four different clocks to time an event. Which recorded time is the most precise?

31.256 s

Explanation:

31.256 seconds is the most precise because it has the most significant figures among them.

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 =