Multi-choice problem: Continue writing the next sentence in this paragraph:

How to get freckles
Understand the cause.
Freckles are a hereditary trait caused by an uneven distribution of skin pigmentation. A freckle occurs when there is a large concentration of the melanin pigment underneath one spot on your skin.

OPTIONS:
a). Melanin pigmentation can be caused by one of two factors: a combination of genetics and physical activity. Genetics and physical activity both affect freckles.;
b). Most natural freckles are small and essentially harmless. They tend to show up on areas exposed to sunlight, like your face, and these are probably the type you're longing for.;
c). Freckles can cause the skin to lighten, or change colors from a normal shade to a darker shade. Freckles often have no identifiable cause.;
d). Freckles will often become noticeably darker if your skin reflects too much melanin. Being curious about freckles can help you have a good look at your freckles and diagnose the underlying causes.;
b).