Tom creates an atomic domain for the credit card number, identifying this as a character field (char 19).

Tom为信用卡号创建一个原子域，并将其标识为字符字段(CHAR 19)。

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Potassium: chemical element, one of the alkali metals, chemical symbol k, atomic number 19.

钾：一种碱金属化学元素，化学符号k，原子序数19。

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