As observed from the top of a lighthouse, 42 m high above sea-level, the angle of depression of a ship sailing directly towards it changes from 30° to 45°. The distance travelled by the ship during the period of observation is:

$42(1-\sqrt{3})$ $42(\sqrt{3}+1)$ $42(\sqrt{3}-1)$ 42

$42(\sqrt{3}-1)$