With no explanation, label each line with "O", "organization-company", "art-broadcastprogram", "art-film", "art-music", "art-other", "art-painting", "art-writtenart", "building-airport" or "building-hospital" preceded by ":".
When
Metro
International
Airways
existed
,
its
headquarters
were
at
the
airport
.
When:O
Metro:organization-company
International:organization-company
Airways:organization-company
existed:O
,:O
its:O
headquarters:O
were:O
at:O
the:O
airport:O
.:O