Isochrone
Does anyone know the answer to this ATPL question and why?
When a course is plotted at minimum time route, one passes from the air isochrone to the corresponding ground isochrone by applying to point K (original spot) a vector KK' which is equal to:
a. wind at K.
b. wind at K'.
c. mean wind from the preceding ground isochrone.
d. mean wind up to the next ground isochrone.