Feet first ascents can occur due to what entering the feet of the dry suit?

Feet first ascents can occur due to the inflation of the dry suit with air, which is the correct choice. When a dry suit is inflated with air, it becomes buoyant, and if this buoyancy is concentrated in the feet area, it can lead to a situation where the diver starts to ascend feet first. The air in the suit may rise toward the feet, especially when there is an imbalance in the distribution of gas within the suit.

This phenomenon often occurs when divers are not properly weighted or if they are ascending too quickly, causing the air to gather at the lower part of the suit. This can inadvertently lead to an uncontrolled ascent, which can be dangerous.

The other options do not create the same buoyancy effect. For instance, helium is used for deep diving due to its low density and does not cause the same issue unless used incorrectly in the dry suit context. Water entering the suit would create additional weight, leading to a negative buoyancy situation rather than a feet first ascent. Similarly, oxygen, while breathable and necessary for diving, does not contribute to buoyancy in the same manner as air does within the suit.