单选题 Many deep-sea divers are lowered and raised by a crew on the surface and receive their oxygen supply through tubes to the surface. Skin-divers use an independent system called scuba, self-contained underwater breathing apparatus. Open-circuit scuba equipment allows bubbles of air to come to the surface. Navy skin-divers use a closed or semi-closed circuit apparatus. With this type of apparatus, the enemy cannot detect the diver's presence because ______