Given a scientific question, generate a correct answer to it.

Ex Input:
Exemplified by cesium, which ignites spontaneously in air, pure elements with a high metallic character tend to be very what?

Ex Output:
reactive

Ex Input:
The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by what?

Ex Output:
bacteria

Ex Input:
What is the leading cause of skin cancer?

Ex Output:
sunlight