This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to know if you have a cavity
Know that cavities are holes in your teeth.
These can be visible, but they may not be. These holes in your teeth are caused by tooth decay.

[a]. Untreated, they can cause a great deal as pain as well as damage to your teeth, bones, gums, and even make you very ill. If they become infected, you will need to seek medical attention to prevent abscesses and the spread of infection..
[b]. Also, dentists often consider cavities to be small enough to fit through the tooth. Cavities are not painful, but can be hard to repair by moving around..
[c]. There is evidence to support the formation of cavities, but dentists strongly disagree whether they are the problem. These holes are also true for cavities under the gum line..
[d]. Cavities are often located on the roof of your mouth. Cavities can also be found on your gums, under your tongue, or anywhere around your upper lip..
The answer is:
[a].