Given a scientific question, generate a correct answer to it.

Ex Input:
Carboxylic acids have an acidic hydrogen atom, but esters do not. what do esters have in place of an acidic hydrogen atom?

Ex Output:
hydrocarbon group

Ex Input:
In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute what?

Ex Output:
urine

Ex Input:
A cartilaginous joint where the bones are joined by fibrocartilage is called a what?

Ex Output:
symphysis