Solving a Question: I Thought it Was D!

[Peter G.]

Hi,

So, I attached the question below.

I thought the answer was D but the book says it is A.

I thought it was D because I thought, well, I don't know, by convention, longest wavelengths and thus shorter frequencies, are to the left.

Thanks!

 

[Peter G.]

So sorry it is rotated. My computer refuses to rotate it!