All of the following are considered input barriers to entry except: (a) control of a key raw material by a single firm.(b) the ability to obtain financing for capital projects at more favorable rates than potential competitors.(c) the fact that workers in a particular industry belong to a union.(d) a patent on a specialized type of capital that is needed to produce a particular product

(C) the fact that workers in a particular industry belong to a union.

Explanation:

All other options are considered as possible entry barriers because they may put hindrance for an entrepreneur to enter the field of producers . The fact that workers of a particular industry belong to a union because role of union comes only when industry starts functioning . It can not play any role at the start  of an industry.

answer; sensory adaptation;