The minimum speed with respect to air that a particular jet aircraft must have in order to keep aloft is 300 km/hr. Suppose that as its pilot prepares to take off, the wind blows eastward at a ground speed that can vary between 0 and 30 km/hr. Ignoring any other fact, a safe procedure to follow, consistent with using up as little fuel as possible, is to
(a) take off eastward at a ground speed of 320 km/hr.
(b) take off westward at a ground speed of 320 km/hr.
(c) take off westward at a ground speed of 300 km/hr.
(d) take off westward at a ground speed of 280 km/hr.