21.

In C++ templates, "typename" (or "class") declares a type template parameter in the template declaration, while "template" is used to declare template template parameters—parameters that accept other templates. Option A correctly distinguishes their roles: typename declares parameters, and template is needed when passing templates as arguments.

Step-by-step Derivation:
Consider template declarations: 'template' uses typename to declare a type parameter. For template template parameters, you use 'template<template class C>' where 'template' indicates the parameter itself is a template. Option A correctly identifies typename's role in parameter declaration and template's role in template parameters. Options B reverses these roles (incorrect). Option C claims they're interchangeable (false—they serve different purposes). Option D is incorrect because both keywords are essential in their respective contexts.