4. Identify Problems A student is describing a longitudinal wave in his notebook. He
writes down "The distance between crests is 3 cm." What is wrong with what the
student recorded?

 "The distance between crests is 3 cm." 

Explanation:

If he writes down "The distance between crests is 3 cm." 

That means he is describing the wavelength of a wave and not longitudinal wave. He ought to write something about " direction "

Longitudinal waves are waves in which the displacement of the medium is in the same direction as, or parallel to, the direction of propagation of the wave. While

Wavelength is the distance between the two successfully Crest or trough

the answer is f1 and d2 because if you put equal force on those two points the beam would balance trust me on this