Cardinal (Catholicism):

For a period ending in the mid-20th century, long-serving cardinal priests were entitled to fill vacancies that arose among the cardinal bishops, just as cardinal deacons of ten years' standing are still entitled to become cardinal priests. Since then, cardinals have been advanced to cardinal bishop exclusively by papal appointment.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". How have cardinals been advance to cardinal priests?
unanswerable