An aeroplane flies along a straight line from A to B with a speed v₀ and back again with the same speed v₀. A steady wind v is blowing. If AB = 1 then
a) total time for the trips is $\frac{2{\mathrm{v}}_0\mathrm{l}}{{\mathrm{v}}_0^2-{\mathrm{v}}^2}$ if wind blows along the line AB
b) total time for the trip is $\frac{2\mathrm{l}}{\sqrt{{\mathrm{v}}_0^2-{\mathrm{v}}^2}}$, if wind blows perpendicular to the line AB
c) total time for the trip decrease because of the presence of wind
d) total time for the trip increase because of the presence of wind