Complete the sentences with the correct verb form.

1. Lightvessels (to moor) in deep waters so that the mariners can rely on them in places where the permanent lighthouse is impracticable.

2. Whistle and horn buoys are similar in shape to a bell buoy, but they carry a whistle that  (to activate) by the rise and fall of the buoy on the seaways, or horns that are sounded at regular intervals by mechanical or electrical means.

3. Sound signals provide information on how  (to avoid) danger and navigate safely.

4. Safe Water Mark is often used to indicate the beginning of a marked channel when  (to approach) from seaward.

5. Special marks  (not to play) any major role in facilitating mariners in safe navigation as they only point out areas of certain interests to mariners.