The sigmoid function is given by

(a) Write a python function that returns the sigmoid function and plot it.

(b) Write a function that returns the derivative of s(x). Note that s(x) = s(1 - s).

(c) Write a function that returns either s(x) or s(x), but not both, depending on how the function is called. Hint: One way to do this is by using a keyword argument.