Select the correct answer from each drop-down menu . a hydrogen fuel cell is more efficient than a combustion reaction because very little of the released energy is when used at home, a fuel cell eliminates the need for thr cell is portable heat electrical energy chemical energy oxygen hydrogen an electrical grid

Heat

Electrical grid

Explanation:

A hydrogen fuel cell works by converting the chemical energy of hydrogen and oxygen to electricity with the means of redox reactions reactions. It has an efficiency in the range of 40–60%. This means only 60-30% of the energy is lost as waste heat.

The mechanism of with which the fuel cell works does not require electricity so an electrical grid would not be required.

answer: the rock sample is a sedimentary rock.

explanation:

it is given that:

mass of rock sample, m = 2.7 g

volume of rock sample,

so, density of rock sample will be

since, the density of rock sample is . hence, the rock sample is a sedimentary rock, whose densities varies between .

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 =