We started pitching the highest camp that has ever been made. Everything took five times as long as it would have taken in a place where there was enough air to breathe; but at last we got the tent up, and when we crawled in, it was not too bad. There was only a light wind, and inside it was not too cold for us to take off our gloves. At night most climbers take off their boots; but I prefer to keep them on. Hillary, on the other hand, took his off and laid them next to his sleeping bag.
They took a long time to finish the work because

Correct Option (B)

The passage explicitly states, "Everything took five times as long as it would have taken in a place where there was enough air to breathe." This sentence directly attributes the extended duration required to complete the work (pitching the tent) to the insufficient oxygen levels at high altitude, making the lack of air the primary cause of the delay.

Incorrect Options:

Option (A) "they were very tired": While exhaustion is a common consequence of high-altitude exertion, the passage does not cite tiredness as the specific reason for the prolonged work duration. The text directly provides another cause.

Option (C) "it was very cold": The passage mentions the cold, noting that "inside it was not too cold for us to take off our gloves." This indicates that while cold was present, it was manageable and not presented as the reason for the work taking five times longer. The delay is not attributed to the temperature.

Option (D) "it was very dark": The provided text does not contain any mention of darkness as a factor affecting the time taken to complete the work. Therefore, this option is not supported by the passage.