In the 3rd paragraph, "... in the regions where it could be practiced... ", here, "it" refers to ______.

A．Fire and the domestication of animals make agriculture possible.
B．Picture language and oral language might appear at the same time.
C．Gunpowder and marine's compass are as important as speech.
D．Language and communication are the most important factors for human development.

A．is the most important step man has ever made
B．is only less important than the domestication of animals
C．and fire are of the same importance
D．can be ranked in importance with the invention of machines