The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field.
D is the correct answer!!
Explanation:
Recall that
so the speed of the bacterium is

Next, we convert this speed to km/hr. Recall that


Therefore,

