The four sentences labelled A, B, C and D, given below in jumbled order, together form a coherent paragraph. Arrange the sentences in the most logical order and choose the sequence that produces the best paragraph.

A. Officials now plan to extend the drone service to two more valleys before the next monsoon.
B. Last year, the district health department therefore began trial flights with small cargo drones.
C. In several Himalayan districts, landslides regularly cut off the only road to remote clinics for days at a time.
D. In the trials, a drone carried vaccines and emergency medicines across a valley in under twenty minutes, a journey that takes five hours by jeep.