Which sentence shows that maglev trains create specific challenges?
A
They go up to 600 kilometers per hour
(around 370 miles per hour).
B
Maglev trains are moved by electric
motors located in the guideway.
С
These magnets use less electricity than
conventional electromagnets, but they
must be cooled to temperatures as low
as minus 267 degrees Celsius (minus 450
degrees Fahrenheit).
D
This raises the train off the track a few
centimeters into a "floating" position.

Ireally don’t know

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

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.