—What do you think of genetically modified food (转基因食品)?

—We all think ______ is careless ______ some scientists to say it is safe before doing any experiment.

A．that, for

B．it, for

C．it, of

D．that, of

根据数据库的规范化理论，下列命题正确的是( )。

A．若R∈2NF，则R∈3NF

B．若R∈3NF，则R∈BCNF

C．若R∈3NF，则R∈2NF

D．若R∈BCNF，则R∈4NF