Nouns, Gender, and the Verb Būti

Stalas is masculine and knyga feminine. What follows from that?

• ANothing beyond the noun itself; gender is a label in the dictionary.
• BThe adjective describing the noun and the pronoun replacing it must follow its gender.
• CTables are thought of as male and books as female.
• DThe two nouns belong to different declensions and cannot be used together.