Procedural abstraction requires the author of a function and the client (user) of that function to know and not to know certain things. Remark on who needs to know and who need not know each of the following items

a) the requirements on the parameter values,
b) exactly what the return value is to be,
c) the implementation details, and
d) the details of how the function is used by the client.

a) Both author and client must know the requirements on the parameter values
b) Both client and author must know what the return value is to be,
c) Only the author needs to know the implementation details.
d) Only the client needs to know the details of the use to which the function is put.