If we were writing, we'd use commas, parentheses, or even dashes to separate this additional information and mark it off as an independent thought group.
He had never seen a wizard work things out like this, simply by looking and touching; but Harry had long since learned that bangs and smoke were more often the marks of ineptitude than expertise.
After each question there will be a pause. During the pause, you must read the four choices marked A), B), C) and D), and decide which is the best answer.