Directions: The first and the last sentences of the passage are numbered 1 and 6, respectively. The rest of the passage is split into four parts and named A, B, C and D. These four parts are not given in their proper order. Read the sentences and find out which of the following combinations is correct.

1. Extinction lacks a clean defining moment with the question arising whether a taxon is extinct when we cannot find an example anymore or when it somehow ceases to be ecologically important. A. Unless there's a plan to rewrite ecological 'rules' wholesale, a successful de-extinction necessarily entails creating an 'invasive' species that can spread to occupy suitable habitat. B. The latter merely moved the goalpost, but by many possible versions of that criterion, most 'endangered' species are 'functionally' extinct already. C. It may only take a technician and bit of labware to create embryos, but if every other hurdle is cleared, providing habitat for a population is another matter altogether. D. So, too are some very rare ones.
2. It is easy to focus on the next step, but the endgame is the mammoth in the room.