What do spf sunscreen and ozone in the earths atmosphere do

If anyone would be able to me out with these three questions it would be these are from the chem 2202 course.

Astring vibrates with a frequency of 10 hz. why can't a person hear the sound waves produced by the vibrating string, no matter how large the amplitude of the waves? out! this is homework and due tomorrow! you so much!

What methods could you use to solubilize calcium carbonate