A vertical line with a “chicken leg” to the entity represents the ________ cardinality symbol.
A. mandatory many
B. mandatory single
C. optional many
D. optional single
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A circle with a “chicken leg” to the entity represents the ________ cardinality symbol.
A. mandatory many
B. mandatory single
C. optional many
D. optional single
Considering the following tables, ________ is a primary key.
A. Class Table: PID
B. Professor Table: Name
C. Student Table: Expected Graduation Year
D. Student Table: ID
《进口许可证》原则实行“一批一证”制度，非“一批一证”的商品发证机关在签发进口许可证时必须在“备案”栏注明“非一批一证”字样，该证在有效期内最多可使用：（ ）。
A. 12次
B. 8次
C. 6次
D. 4次
进口货物许可证的有效期一般是：（）。
A. 6个月
B. 12个月
C. 3个月
D. 9个月
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