What does it mean to say that mass is conserved during a physical change?

Although longitudinal waves can travel through all media types what is the disadvantage of this type of wave transmission? a) he efficiency of he wave transmission is greatly diminished as the media becomes less dense b) the oscillation of one set of particles has a huge impact on neighboring particles c) particles become too close together as they bump into each other making it more likely to hit other particles down the line

The discovery of the electron as a subatomic particle was a result of

Which basic property determines a colored light's placement on the spectrum of visible light? a. speed b. frequency c. amplitude d. velocity