If a ‘vertical aerial photograph’, (20 cm x 20 cm) in size, on a R.F.I: 10,000, has 60% longitudinal overlap and 40% side overlap, the actual ground length covered by each photograph in the longitudinal direction of the flight will be : (a) 4 km (b) 6 km (c) 0.8 km (d) 0.4 km