Final answer:
Scuba divers must ascend at a slow speed of 10 to 20 feet per minute to avoid decompression sickness by allowing dissolved gases in the blood to escape gradually.
Step-by-step explanation:
The question concerning the scuba diver's elevation change during the ascent can be found under the Mathematics subject, as it requires an understanding of units of measure and rate of change.
Based on the information provided, to avoid decompression sickness (DCS), divers must ascend from depths at relatively slow speeds of 10 or 20 feet per minute. This safety measure ensures that dissolved gases have time to escape the diver's blood gradually, avoiding DCS symptoms caused by too rapid an ascent.