An electric kettle has two coils of same power . When one coil is switched on , it takes `15 min` to boil water , and when the second coil is switched on , it takes `30 min`. How long will it take to boil water when both the coils are used in `i`. Series and `ii`. parallel?

A. `9:2`

B. `2:9`

C. `4:5`

D. `5:4`