阅读理解。

     Fish have ears. Really. They' re quite small and have no opening to the outside world carrying sound

through the body. For the past seven years, Simon Thorrold, a university professor, has been examining

fish ears, small round ear bones called otoliths.

What can we learn about fish ears from the text?

A. They are small soft rings.

B. They are not seen from the outside.

C. They are openings only on food fish.

D. They are not used to receive sound.