List three primary structures that make up the placenta?

How do the uv rays cause damage?   what exactly causes the skin to burn;   can you be more specific about this mechanism?   3. how do photosensitive medications increase a person's sensitivity to sunlight and therefore increase their risk of getting burned?     what kind of medications can increase photo-sensitivity, can you think of some examples?

How can a driver best determine the safest speed to drive?

If an online school program (i.e k12, ecot) assigns me a computer, would it be ok to use social media on it? would i get in trouble? i am not really sure from reading their terms and such.