题目内容

()于 1973 年建立了 UPC 代码系统，并全面实现了该码制标准化。

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条码符号技术的主要内容是研究各种()、符号表示以及符号制作。

UPC 的中文意思是()。

Shanghai is one of the largest cities in the world that______large populations each over ten million.

A. has
B. have
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When I try to understand______that prevents so many Americans from being as happy as might

A. why it does
B. what it does
C. what it is
D. why it is