Read the following paragraph carefully and choose the best topic sentence from the four possible answers that follow the paragraph by writing A, B, C or D. 

Data from both Russia and the United States show Arctic sea ice extended to about 4.2 million square miles in June, the lowest ice coverage recorded for that month in the satellite data record, which began in 1979. If the melting trend continues, sea ice levels could fall by September to less than 1. 6 million square miles, the lowest recorded in 2007. The fast pace of the melting could mean that the North Pole could be ice-free during the summer within just a few decades, far sooner than previously estimated. Arctic waterways（航道）typically choked（堵塞）with ice are now opening up.