Read the passage below and choose one correct answer for each question.

The primary attraction of snorkeling is the opportunity to observe underwater life in a natural setting, such as coral reefs, fish, starfish, and mollusks. Other organisms that can be seen while snorkeling include various forms of seaweed, jellyfish, shrimp and sea turtles. Snorkeling requires no special training, only the ability to swim and to breathe through the snorkel. However, it is considered advisable that one get some instruction from a tour guide, dive shop, or equipment rental shop, any of which often can be found around popular snorkeling locations. Instruction generally covers equipment usage, basic safety, what to look for, and what to look out for, including how not to damage fragile organisms such as coral. As with scuba diving, it is always recommended that one, should not snorkel alone, but rather with a friend, a guide, or a tour group.

Swim fins used in snorkeling are usually longer than those used in diving. Snorkel is a tube about thirty centimeters (twelve inches) long, usually J-shaped, fitted with a mouthpiece, and constructed of rubber or plastic. It is used for breathing air from above the water surface when the mouth and nose are submerged, either when snorkeling or during a surface swim before or after scuba diving. The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask, as sticking the snorkel in between the strap and the mask could cause the mask to leak, or risk losing the snorkel should the diver choose to switch to scuba.

Typically, the diving mask also serves to prevent breathing through the nose, so that one is forced to breathe through the snorkel. This also provides some negative pressure which helps keep the .mask sealed against the face, though attempting to breathe out through the nose can break this seal and fog the mask.

The snorkel ________.