能促进花卉成熟的必要元素为（）
A.氮
B.钙
C.磷
D.镁
参考答案：C
--- He has never traveled to a foreign country, ?
--- Yes. He has been to England once.
A．does he
B．doesn't he
C．has he
D．hasn't he
技术性贸易壁垒的形式不包括______
A．技术标准 B．卫生检疫制度C．进口许可证制 D．包括制度
