Which of the following statements regarding caries depth is correct?

a. Incipient caries penetrates less than halfway through the enamel.
b. Moderate caries penetrates through the enamel and the DEJ.
c. Advanced caries penetrates all the way through the enamel, the dentin , and to the pulp.
d. Severe caries penetrates more than halfway through the enamel but not the DEJ.

a. Incipient caries, also called "enamel caries," is the first stage of existence.