Discrimination.

No public utility shall, as to rates or services, make or grant any unreasonable preference or advantage to any corporation or person within any classification or subject any corporation or person within any classification to any unreasonable prejudice or disadvantage. No public utility shall establish and maintain any unreasonable differences as to rates of service either as between localities or as between classes of service. Nothing shall prohibit, however, the commission from approving: A. economic development rates; B. rates designed to retain load; C. rates and programs designed to reduce the burden of energy costs on low- income customers; and D. energy efficiency programs designed to reduce the burden of energy costs on low-income customers pursuant to the Efficient Use of Energy Act [Chapter 62, Article 17 NMSA 1978].