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. Through the last century, however, most of them were filled with rubble and sewage and forgotten behind new construction.
B. Volunteer groups in several cities have now begun clearing and restoring these structures, one well at a time.
C. Medieval stepwells once supplied Indian towns with water through the driest months of the year.
D. The restored wells are already recharging groundwater and drawing visitors, proof that old engineering can answer new problems.