Complex sentence: Constant Elasticity of Transformation (CET) is firstly brought forward by Powell and Gruen (1968), Powell and Gruen (1968) which is a new form of production-possibility frontier.
Split into simpler sentences: Constant Elasticity of Transformation (CET) was first advanced by Powell and Gruen (1968), Powell and Gruen (1968). It is a new form of production-possibility frontier.