The circuit below contains an ideal battery, four identical light bulbs, and a switch. The switch is initially open.
E3-EC1 Prelab Q5-Fig_web
Rank the bulbs according to brightness, from brightest to dimmest, when the switch is open.
Give a complete ranking using only the bulb letters and the symbols '>' and '='. If any bulbs do not light, indicate this explicitly by setting the bulb letter equal to 0 in the ranking, for example A=B>X>Y=Z=0)

The question is incomplete! circuit figure is attached below and answer and explanation is provided below.

Bulb_A = Bulb_B = Bulb_D and Bulb_C = 0.

Explanation:

What happens when switch is open?

When the switch is open Bulb_C is open circuited meaning that there is no way for the current to flow through it. This path offers infinite resistance to the current therefore, current will try to take a least resistance path that is through Bulb_B.

So eventually, when the switch is open the circuit becomes a simple series circuit with path From battery to Bulb_A to Bulb_B to Bulb_D to battery with Bulb_C = 0.

What happens in a series circuit?

We know that in a series circuit, there is only one path for the current to flow therefore, same current will flow through all the series Bulbs and their brightness will be same. Bulb_A = Bulb_B = Bulb_D

Brightness in a series circuit:

We also know know that in a series circuit, resistance gets summed up and voltage across each Bulb gets shared which results in less power dissipation that's why Bulbs connected in series appear dimmer as compared to when they are connected in parallel.

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 =