When Greek messenger Pheidippides burst into Athens in 490 BC, having run the first marathon, he probably never imagined his superhuman effort would one day be considered a race. For a growing number of athletes, 26.2 miles (42.19km) is not enough. 
        The rise of ultramarathon running has been unstoppable in the 21st century. Steve Diederich, the founder of global race database Run Ultra, says 12 years ago he had around 60 events listed. Now he estimates there’s somewhere between 2,200 and 2,300. Set in a national conservation area, the Al Marmoom Ultramarathon claims to be the longest desert ultra in the world. The 270km (168 mile) route was run over five days in December in temperatures as high as 35 degrees Celsius (95 degrees Fahrenheit).
        This year, 19 runners (four women) completed the race, with 36-year-old Moroccan Rachid El Morabity crossing the finish line first in 31 hours and 17 minutes. Eleven runners didn’t make it, as well as many others in the shorter 100 km and 50 km versions of the race.
        Race manager Ole Brom took care of the health of the athletes. Running these distances across the sand is really an extreme sport. “On the first day after about 40 km, about 12 km from the end, one athlete collapsed unconscious,” says Brom. “He ignored the signs of dehydration and he suffered the consequences.”
        Running across the dunes was not without its rewards. “It’s really peaceful,” says 45-year-old female race winner Magdalena Boulet, “you can’t really see anything for miles and miles.” “It’s wonderful,” Boulet adds. “On certain routes there were Oryx, there were sand gazelle, mountain gazelles, which was really beautiful. We saw eagles and a lot of different migrating birds.” As a conservation area, runners were punished for dropping trash and required to bury human waste.

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1. What does the underlined word in Paragraph 4 mean in Chinese?（词义猜测）

A    “障碍”.

B    “干旱”.

C    “脱水”.

D    “缺氧”.