Question 2 What causes a sound wave to travel?
A
An object vibrates, and those vibrations carry air in all directions as transverse waves.
O
B
An object widrates, and those vibrations cany air in all directions as long dudinal waves.
с
An object vibrates, and those vibrations travel through air in one direction as a transverse wave
D
An object vibrates, and those vibrations travel through air in one direction as longitudinal waves.

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.