30) B

Let us start with the flat section which was 1 km long.
Runner’s speed = 6km/h
Time = 1/6 x 60 = 10 minutes
Cyclist speed = 30km/h
Time = 1/30 x 60 = 2 minutes
Difference = 10-2 = 8 minutes

Only options B and C show that the cyclist is ahead by 8 minutes. This rules out options A and D

Muddy section which is 1.5 km long
Runner’s speed: 4km/h
Time = 1.5/4 x60 = 22 minutes 30 seconds
Total time for runner = 32 minutes 30 seconds
Cyclist speed: 3 km/h
Time = 1.5/3 x 60 = 30 minutes
Total time for cyclist so far = 32 minutes

This shows that the cyclist is still ahead by 30 seconds after 2.5 km of the whole race.
Only B shows that. C shows that the runner gets ahead of the cyclist during the muddy race which is not true.

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