【单选题】

The defense works __________ long ago to keep the enemy away.

A. were built
B. has been built
C. had been built
D. was built

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根据网考网移动考试中心的统计，该试题：

11%的考友选择了A选项

10%的考友选择了B选项

1%的考友选择了C选项

78%的考友选择了D选项