Expression of Ideas — module quiz

The Svalbard Global Seed Vault holds duplicate samples of crop seed sent from gene banks around the world, and it was cut a hundred metres or so into an Arctic mountainside partly so that the permafrost around it would keep the collection frozen even if the refrigeration and the power both failed. In the autumn of 2016 that permafrost thawed further than the design had allowed for: meltwater ran into the entrance tunnel, froze inside it, and staff arriving with new deposits found the corridor glazed shut. ______, the water reached no further than the tunnel; the storage chambers beyond it stayed dry, and no depositor was reported to have lost a sample.

Which choice completes the text with the most logical transition?