In the Theory of Balanced Growth, what is considered the primary cause of underdevelopment?

Given below are two statements, one labeled as Assertion (A) and the other as Reason (R): Assertion (A): According to Paul Romer, public and private investment in human capital generate external economies. Reason (R): Investment in human capital leads to improvement in productivity and thereby offsets the natural tendency of diminishing returns. In light of the above statements, choose the correct answer from the options given below:

Match List I (Book) with List II (Author) and select the correct combination: List I: (a) Theory of Economic Development (b) Growth and Underdevelopment (c) Strategy of Economic Development (d) Capital Formation in Underdeveloped Countries List II: (i) Schumpeter (ii) Hirschman (iii) Hirschman (iv) Nurkse