Electric current is the flow of charged particles called blank .

The electric current is the flow of electrons which are negatively charged particles.

 Explanation:

The flow of electrons in a circuit is called the electric current. It is measured with one coulomb of charge across the circuit in 1 second that is called an ampere. This ampere is the standard unit of electric current.

The flow of electrons across the circuit is achieved by the potential difference across the circuit. This potential difference is the difference of electrical voltage generated across the circuit in order to achieve flow of current.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.