The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

All those who have set foot in Antarctica agree that its main and most cruel characteristic is wind. When we wintered in Adelie Land the wind on 5th July blew nonstop for eight hours at an average speed of 170 mph; gusts were recorded of over 150 mph, and the average wind speed for the month was 63. 3. In these conditions it was impossible to stand for more than a few seconds. 

The biggest （） to life in Antarctica is wind.