Why quantity refractive index dooesn't have unit

Refractive index is expressed as a ratio of speed of light in vacuum relative to the speed of considered medium. As it is a ratio and the units m/s or km/h cancel out each other. Therefore, the qauntity refractive index does not have any unit. I hope it will be helpful to you. Thank you

when a device is used in a circuit in which the voltage is 81 v, the current flowing through the device is 3 a. what is the resistance of the device?

a. 27 ω

b. 40.5 a

c. 243 ω

d. 243 a

the correct answer is a. 27