The four sentences (labelled $1, 2, 3$ and $4$) given below, when properly sequenced, would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer.
- Veena Sahajwalla, a materials scientist at the University of New South Wales, believes there is a new way of solving this problem.
- Her vision is for automated drones and robots to pick out components, put them into a small furnace and smelt them at specific temperatures to extract the metals one by one before they are sent off to manufacturers for reuse.
- E-waste contains huge quantities of valuable metals, ceramics and plastics that could be salvaged and recycled, although currently not enough of it is.
- She plans to build micro factories that can tease apart the tangle of materials in mobile phones, computers and other e-waste.