#6 - Lower frequencies correspond to <em>longer</em> wavelengths.
#7 - To change the pitch of a sound, you have to change its <em>frequency</em>.
Explanation:
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The relationship between the speed, the frequency and the wavelength of a wave is given by:

where
f is the frequency
v is the speed of the wave

is its wavelength
for the pressure wave in this problem,

and v=1400 m/s, therefore its frequency is
Answer:The frequency of the echo is slightly decreased
Explanation:
Given
speed of boat 
cliff is
away
when boat is still , suppose t is the time taken by the echo to reach observer on the boat
But as soon as boat starts moving the distance between cliff and boat decreasing and time for echo to reach observer also decreases
and we know 
therefore frequency of the echo slightly decreased.
Answer:
B) converging tectonic plates
Explanation: