Today, the exchange rate between pounds and Australian dollars is approximately AUD 1 = GBP 0.6400, whereas four years ago, the exchange rate was about AUD 1 = GBP 0.4200. Over the past four years, the average annual percentage change in the value of the Australian dollar against the pound was closest to:

(A) - 52.4% pa (B) - 11.1% pa (C) + 11.1% pa (D) + 52.4% pa