Compare the statements $A$ and $B$.
$\text{Statement } A$: When the urine moves through the descending limb,it becomes hypertonic,and as it passes through the ascending limb of Henle's loop,it becomes hypotonic.
$\text{Statement } B$: The descending limb is permeable to sodium ions,while the ascending limb is impermeable to sodium ions.

• A
  Statement $A$ is correct and $B$ is incorrect.
• B
  Statement $A$ is wrong and $B$ is correct.
• C
  Both statements $A$ and $B$ are incorrect.
• D
  Both statements $A$ and $B$ are correct.