A heating coil can heat the water of a vessel from \[20{}^\circ C\] to \[60{}^\circ C\] in 30 minutes. Two such heating coils are put in series and then used to heat the same amount of water through the same temperature range. The time taken now will be (neglecting thermal capacity of the coils) [MP PMT 1997]

• A.            60 minutes                            
• B.            30 minutes
• C.            15 minutes                            
• D.            7.5 minutes