In the following passage some words have been deleted. Fill in the blanks with the help of the alternatives given.

The safety of ships at sea does not depend only on the arrangement made in the ships themselves. Dangerous (1)........ and rocks may be seen in the (2)......... but they are a threat to safe (3)........ at night. Lighthouses are therefore (4)......... at such places to warn sailors of the (5)...... Modern lighthouses are put up at all such (6)........ near the courses followed by (7)...... on some rocky coasts several lighthouses may be (8)........ not far from each other, and in order that the (9)........ on a ship may (10)....... which is which, they have different arrangements of light.