The Overpriced Petroleum Extraction Company (or OPEC) has just declared its acquisition of some small firms with facilities which will permit OPEC to process oil via the whole refining procedure, from oil field recovery via transporting and then trading the refined petroleum products. Such acquisitions are an illustration of: (1) Horizontal integration. (2) Inverse integration. (3) Diagonal integration. (4) Vertical integration. (5) Conglomerate integration.
Find out the right answer from the above options.