The sentence below from the section "Changing Food To Fuel" helps to prove the claim that digestion is an involuntary process. Chewing involves voluntary muscles — these are ones that we consciously control — but once we swallow, digestion becomes involuntary.

Which sentence from the section provides further support for the claim?

A
Our teeth start the process by chewing and grinding up each mouthful of food.
B
The tongue works it into a ball-shaped mass, or bolus, for swallowing.
C
Saliva uses specialized proteins called enzymes.
D
It is sealed in the stomach by two ringlike sphincter muscles for several hours.