What materials are poor thermal insulators?

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.

What is the force that opposes motion and works against the downward pull? a) friction b) gravity c) weight d) acceleration

This is the number of complete movements of a wave per second.