A cyclist drove one kilometre, with the wind in his back, in 3 minutes and drove the same way back, against the wind, in 4 minutes. If we assume that the cyclist always puts constant force on the pedals, how much time would it take him to drive 1 km without wind ?

• A. $2\frac{1}{3}$$ min
• B. $2\frac{3}{7}$$ min
• C. $3\frac{3}{7}$$ min
• D. $3\frac{7}{12}$$ min