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3、设F是基本关系R的一个或一组属性，但不是关系R的码，如果F与基本关系S的主码Ks相对应，则F是R的（），基本关系R成为（），基本关系S称为（）。A、主码B、外码C、参照关系D、被参照关系或目标关系

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简述参照完整性：

Situated at the southern tip of the _, Hangzhou was the capital of the Wuyue State of the Five Dynasties and Ten Kingdoms period and also the capital of the _.

Hangzhou's _______is famed for its fragrance, luscious flavour, pleasant leaf shape, and emerald colour.

In the Yuan dynasty, Italian traveller Marco Polo (125 —1324) was impressed by the beauty of _______ and lauded it as "the most beautiful and elegant city in the world".

A. Nanjing
B. Hangzhou
C. Beijing
D. Shanghai