Which of the Following Is a Defining Principle of Producer

Which of the following is a defining principle of producer co-operatives, compared to other types of worker ownership?

A) Governments must be involved, in order to be successful.
B) Unions must be included in all decisions made.
C) Each co-op member owns one share only, and non-members cannot own shares.
D) They are not allowed to grow beyond a certain size, 25 members, for example.