Ahelicopter is hovering motionless above a disabled boat, while rescue workers use a rope to lift an injured sailor. while that sailor is being lifted upward, the net force on the motionless helicopter is(a) downward and equal to the sailor's weight. (b) zero. (c) upward and equal to the sailor's weight. (d) downward and equal to the sailor's weight plus the helicopter's weight.

The keyword for the solution of this problem is the word “motionless”. We are told here that the helicopter is motionless, this means (according to Newton’s 1st law of motion) the net force on the helicopter is zero.

Now, if we want to go further in the analysis, we can draw a Free Body Diagram (FBD) of this situation (figure attached), assuming there are no forces in the x-axis.

Nevertheless, in the y-axis we have the weight which is equal to the weight of the helicopter plus the weight of the sailor (assuming the rope is weightless):

And the lift force  which is with the same magnitude but in the opposite direction of the weight:

Therefore, the correct option is B.

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 =