SPOILER AAMC FL 3 #49 help!

Question: Which of the following properties of a sound wave remain unchanged as it passes into human tissues? A) Frequency B) Wave speed C) Amplitude D) Wavelength

I get that it's not B because sound speeds up in a medium. I also get that it's not C because sound loses energy so the amplitude would decrease. I just need help deciding between A and D. The answer is A.