Understanding Longitudinal Waves: Regents Exam Question with No Correct Answer

[Qube]

http://min.us/lcyfgi

My physics teacher told the class that this question had no valid answer choice (none of the choices were right).

After further research at home today, I think the answer is (2) - or the wavelength is equal to the distance between A and C, as

In the case of a longitudinal wave, a wavelength measurement is made by measuring the distance from a compression to the next compression or from a rarefaction to the next rarefaction.

http://www.physicsclassroom.com/class/waves/u10l2a.cfm

Help - I have my physics final tomorrow !

 

[Fewmet]

It looks to me like A and C are both descending nodes, which would mean 2 is a correct choice.

A sketch of the corresponding transverse wave leads to the same answer.