For the purposes of this Division, a fire
trail is taken to be
"closed" if--
(a) the trail is wholly or partly removed or destroyed, or
(b) the trail is obstructed so that the trail is not available for use as a fire trail, or
(c) a sign has been placed indicating that the trail is not available for use as a fire trail, or
(d) the trail is otherwise not available for proper use as a fire trail.