What distinguishes a purely ``functional'' programming language from an ``imperative'' one? (a) there are no variables and hence no assignment operation in a purely functional language. (b) a purely functional language lacks the ``go to'' statement, but an imperative language always has such a command. (c) all subprograms must be declared with the keyword function in a purely functional language. (d) there is no real difference, only a difference in the recommended coding style.