"Unitary business" may include, but is not limited to, a business enterprise the activities of which:
(A) Are in the same general line of business (such as manufacturing, wholesaling or retailing); or
(B) Constitute steps in a vertically integrated process (such as the steps involved in the production of natural resources, which might include exploration, mining, refining and marketing).
(c) Whether two or more corporations that are included in the same consolidated federal return are engaged in a unitary business may be determined by making reference to corporations that are doing business in the United States and are subject to federal income taxation, whether or not those corporations are includable in the consolidated return. No other corporations may be taken into consideration in making such a determination, except in a case in which the transactions or relationships between such corporations are made in an attempt to evade or avoid taxation.