They are operated by hydraulic pressure and are used for braking.

They are operated by hydraulic pressure and are used for braking.
A.他们可以用来减速。
B.他们用水压来减速。
C.他们打开水箱用来减速。
D.它们由液压驱动，用于制动。
正确答案：它们由液压驱动，用于制动。