Arrange the sentences in the most logical order to form a coherent paragraph. From the given options (A, B, C, D) choose the most appropriate sequence.
(i) For almost a billion years, while its core churned and produced a protective magnetic field, Mars may have been among the friendlier places for life as we know it to set up and flourish.
(ii) But that is what the $2.4-billion 2020 rover will attempt to do.
(iii) Though Mars is a parched, toxic desert today, the planet was once much warmer and covered in liquid seas.
(iv) The trouble is, finding evidence for past life on an alien world, especially if that extinct life is microscopic, is not exactly easy.