How is the explosive force of a volcanic eruption related to the water content of the magma ?

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

i'd have to say the answer is probable the second option "the geosphere interacting with the atmosphere"

the energy generated by an individual's metabolism is calculated in met (metabolic equivalent of task). the met is the ratio of the work metabolic rate to the resting metabolic rate. one met is illustrated as 1 kcal per kilogram per hour, and is probably equal to the energy cost of sitting quietly.

1 met is equal to 1.162 watt per kg

1 kg is equal to 1000 grams

as the person weighs 80000 gm, that is, 80 kg.

therefore, 80*1.162 = 92.96

thus, the person expends 92.96 watts of power while at rest.