A tow of 9 barges is made up three abreast by three long. The towboat is faced up to the last barge of the center string. The outer two strings of barges are the（）.

A.a mechanical breakdown on the tug
B.adverse tidal current conditions
C.the tug reducing its speed
D.All of the above

A.celestial meridian
B.celestial equator
C.celestial horizon
D.principal vertical circle

A.south celestial pole
B.north celestial pole
C.observer's meridian
D.Greenwich meridian