The distance between a given planet and the sun varies because A. planets get farther and farther away from the sun over time
B. each planet has an elliptical orbital path
C. the sun is constantly moving
D. the sun's gravitational pull decreases at certain parts of the year
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Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current

Iwould think it is temperature, silica, oxides (gases)

The cell reaction uses external supplies of a full and an oxidant, which are consumed and needs to be provided continuously. the cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant.  hope this ! (: