The water main in cities must be placed at a sufficient depth below the earth’s surface to avoid freezing during extended periods of subfreezing temperatures. determine the minimum depth at which the water main must be placed at a location where the soil is initially at 15°c and the earth’s surface temperature under the worst conditions is expected to remain at -10°c for a period of 77days. takethe properties of the soil at that location to be k = 0.7 w/m°c and α = 1.4 x 10-5 m2/s.

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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