Question

acne
a is caused by a bacterial infection.
b is caused by low levels of sebum.
c affects all teenagers.
d all of the above

Explanation:

Acne is often associated with bacterial infection (e.g., Cutibacterium acnes). Sebum levels are typically high in acne - prone skin (not low). And not all teenagers have acne.

Answer:

A. is caused by a bacterial infection.