Why do historians argue that there was a mixture of enterprise, partly

Why do historians argue that there was a mixture of enterprise, partly private and partly governmental, in building our transportation system?

(a) Private individuals took the initiative and were able to extract special privileges and financial assistance from government for the purpose of enhancing their profits.
(b) Political leaders, who wanted transportation improvement for reasons of local or national ambition, took the initiative and coaxed private enterprise into building the transportation
system by the offer of special financial advantages.
(c) The sheer size of the investment required for a transportation system was beyond the capability of private enterprise alone.
(d) All of the above ideas have been used by historians to explain the mixture of enterprise in building our transportation system.

ANSWER

(d)