Given the task definition and input, reply with output. In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".

[header] How to treat mild altitude illness [title] Descend to a lower altitude. [step] If you start to experience any symptoms of mild altitude sickness, such as a headache, lightheadedness, or lung pain, you should descend by at least 500 m or 1 , 600 feet. This will give your body time to acclimatize to the altitude and relieve your symptoms. <sep> (A) [substeps] If you experience brief symptoms such as fatigue or chest aches, you should lower. If you experience movements that become more fluid while you are lower than normal, that form of exertion can aggravate the symptoms of higher altitude illness. (B) [substeps] Don't go too far down. Those below 200 m are at a higher risk for altitude sickness than those below 250 m or 1 , 800 feet. (C) [substeps] You should avoid attempting to go to higher altitudes for two to three days, or until your symptoms have disappeared. Severe altitude sickness can be fatal, so if you notice your symptoms getting worse, make sure you descend to a much lower height. (D) The more acclimatized you are, the greater the chance of moderate to severe altitude sickness. [title] Avoid flying over any giant ice and snow piles.
C