下列哪一项是新企业进入一个行业的自然进入壁垒（）
A.许可证
B.规模经济
C.专利
D.给予公共特许经营权
参考答案：B
It is said that the early European playing-cards ______ for entertainment and education.
A．were designed
B．have designed
C．have been designed
D．were being designed
作为一名中学教师，对于自己任教的学科，应具备哪些学科知识？
