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. Encouraged by the results, the corporation has now ordered twenty more units and begun training former sanitation workers as their operators. B. For decades, blocked sewer lines in the city were cleared by men who climbed into the manholes themselves. C. The machine's cameras and robotic arm clear a blockage while its operator stands safely on the road above. D. That practice began to change two years ago, when the municipal corporation deployed its first sewer cleaning robot.