In January, 2004, the United States successfully launched “Spirit”, a Mars Exploration Rover, a new milestone in the history of mankind.
A. it marked
B. marking
C. marked
D. to mark
网友回答
A. it marked
(答案→)B. marking
C. marked
D. to mark
解析：marking是分词作结果状语,表示”标志着”。