Once from space, the Great Wall is till great in sight.A. seeingB. to seeC. seenD. saw

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A. seeing
B. to see
(答案→)C. seen
D. saw
解析：考查从句的省略式，主从句的主语一致时，可以省略从句的主语和be动词，once seen=once the Great Wall is seen，句意是：一旦从太空看，长城看起来还是很伟大。选C。