单项选择题
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.port and starboard strings B.outer strings C.drag strings D.side strings
单项选择题 A tow can override its tug as a result of（）.
单项选择题 A time diagram is a diagram on the plane of the（）.
单项选择题 A time diagram is a diagram of the celestial sphere as observed from above the（）.
