Question: Read the following conversation and answer the question.  M: Would you please tell me about your best holiday, Nancy?  W: Well, it was my trip in Nepal. We went there 10 years ago, doing some walking in the mountains.  M: How long did you stay there?  W: 20 days.  M: 20 days! That's a long time. How far did you walk every day?  W: Perhaps 15 kilometers. You have to climb up and down a lot every day.  M: So, it was very hard.  W: Yes, very hard. Sometimes, it's difficult to walk in the mountains where the air is very thin, and you have a lot of problems when breathing.  M: What kind of equipment did you take?  W: Not much. Only a bag, a sleeping bag and some warm clothing. We carried all the things by ourselves.  M: And how was the food?  W: There was rice and some vegetables, normally no meat. But it was good.  M: What was the best thing about the trip?  W: The walking and the exercise. I hate sitting in the sun and like to be out in the countryside. The air is very clean and it's very quiet. Nepal is a good place. You must go!  Question: What problem did they have?   - They didn't have modern equipment.  - There wasn't enough oxygen sometimes.  - They couldn't walk freely in the mountains.
Answer:
There wasn't enough oxygen sometimes.