What are two rules for compacting IPv6 addresses？（）
A. The maximum number of times a double colon can replace a 16-bit segment that consists of all zeroes is two
B. The leading zeroes in any 16-bit segment do not have to be written.
C. Every 16-bit segment that consists of all zeroes can be represented with a single colon
D. The trailing zeroes in any 16-bit segment do not have to be written.
E. Any single， continuous string of one or more 16-bit segments that consists of all zeroes can be represented with a double colon
F. Two zeroes in the middle of any 16-bit segment do not have to be writte
出现柏油便时，上消化道出血至少达多少毫升以上（）。
A. 5
B. 20
C. 30
D. 50
E. 100
维护交易公平的意义何在？商法是如何维护这一原则的？
仿形铣削的加工方式，常见的有（）和立体模型仿形两种。
A. 外形
B. 样板轮廓
C. 壳体
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