Only when something gets lost .
A. can you realize how important it is to you
B. how important you can realize it is to you
C. you can realize how important it is to you
D. can you realize how important is it to you
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(答案→)A. can you realize how important it is to you
B. how important you can realize it is to you
C. you can realize how important it is to you
D. can you realize how important is it to you
解析：考查倒装句。Only + 状语，放在句首时，使用部分倒装。从句不倒装，主句倒装，主句后的宾语从句应用陈述句语序。所以选A。