The change in frequency of a wave as its source moves in relation to an observer. A)
Ultrasound
B)
Frequency
C)
Doppler Effect
D)
Echo

B) Frequency

Explanation:

answer: the direction of the wind is expressed as the direction from which the wind is blowing. for example, easterly winds blow from east to west, while westerly winds blow from west to east.

explanation:

c because the action was the roller coaster reaches bottom of hill (action) reaction would be blood tends to drain away from head

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