题目信息
Though most paper currency was at one time backed by fixed assets such as gold or silver, it now derives its purchasing power from a declaratory fiat of the issuing government.
A:was at one time backed by
B:had at one time been backed by
C:was at one time backing
D:had at one time backed
E:has at one time been backed by
参考答案及共享解析
共享解析来源为网络权威资源、GMAT高分考生等； 如有疑问，欢迎在评论区提问与讨论
本题耗时：
已选答案：
正确答案：
A:was at one time backed by
现在完成时的两个条件：1）动作由过去持续到现在 2）动作的effect持续到现在 backed by fixed assets such as gold or silver 不满足以上任何一个条件不能选现在完成时
at one time用过去式
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题目来源
Manhattan
