The alimentary canal of humans is _____ long.

Concept: The alimentary canal is the passage through which food has entered the body. The alimentary canal starts from the mouth and ends at the anus. The digestion process is done with the help of this canal. For the digestion process, the alimentary canal plays an important role. This canal is 6 to 9 meters long. It starts at the mouth through different organs of the body it ends at the anus. The digestion process starts at the mouth when food enters it.  

Hence, the correct answer is option C.