A man runsa distance of \(9 \mathrm{~km} / \mathrm{h}\) for the first \(4 \mathrm{~km}\) and then \(2 \mathrm{~km} / \mathrm{h}\) for the rest of the distance. The whole fun takes him one hour. His average speed for the first \(4 \mathrm{~km}\) is

A. \(6 \mathrm{~km} / \mathrm{h}\)
B. \(8 \mathrm{~km} / \mathrm{h}\)  
C. \(\quad 9 \mathrm{~km} / \mathrm{h}\)
D. \(11 \mathrm{~km} / \mathrm{h}\)

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Correct Answer: B