单项选择题
基于以下叙述：有关系模式P(A,B,C,D,E,F,G,H,I,J)，根据语义有如下函数依赖集：F={ABD→E,AB→G,B→F,C→J,C→I,G→H}
A．1NF B．2NF C．3NF D．BCNF
A．SELECT * FROM sc WHERE grade=NULL B．SELECT * FROM sc WHERE grade IS ‘ ’ C．SELECT * FROM sc WHERE grade IS NULL D．SELECT * FROM sc WHERE grade =‘ ’