When an ambulance passes by, the siren’s pitch changes. The sounds becomes higher-pitched as it approaches, and then the pitch lowers as it gets further away. This change in pitch is known as the Doppler effect. What causes this change in pitch? answers:
A: The waves emitted by an object moving toward an observer are squeezed, and frequency appears to increase.
B: The waves emitted by an object moving away from an observer are squeezed, and frequency appears to increase.
C: The waves emitted by an object moving toward an observer are stretched, and frequency appears to decrease.
D: The waves emitted by an object moving toward an observer are stretched, and frequency appears to increase.

sodium (na), for example, has one electron in its valence shell. this is an unstable state because that valence shell needs eight electrons to be full. in order to fill its valence shell, find a way to add seven electrons to its valence shell

answer is a

explanation:

metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

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what are the statements