a submarine completed a 450km training with an average speed of 50 km/h. For the first 180 km, it traveled at an average speed of 60 km/h. What was the average speed for the remaining distance?

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

If your workout exceeds an hour you should substitute "a sports drink" for water.hope my answer has you : )