Once a falling object has reached a constant velocity, the object a. stops accelerating. b. stops speeding up. c. experiences equal forces of gravity and air resistance. d. all of the above

option D

Explanation:

When an object is falling, its speed increases due to the acceleration due to gravity but after travelling some distance, the air resistance comes into role and then the object attains a constant speed. It means the acceleration of body is zero. It stops speeding up.

It experiences an air resistance force which is equal to force of gravity.

in physics zero. in muscle fatigue could be high

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 =