Which part of the nephron is responsible for secretion?

Transport of electrolytes through the loop of Henle takes place by:

In which of the following regions of a nephron does maximum reabsorption of useful substances take place?

Given below are two statements:
Statement $I$: In the nephron,the descending limb of the loop of Henle is impermeable to water and permeable to electrolytes.
Statement $II$: The proximal convoluted tubule is lined by simple columnar brush border epithelium and increases the surface area for reabsorption.
In the light of the above statements,choose the correct answer from the options given below:

Glucose is mainly absorbed in

Which of the following statement$(s)$ is/are true?
$(1)$ Urine is hypertonic in the distal convoluted tubule.
$(2)$ When the urine passes into the collecting tubule,it becomes hypotonic.
$(3)$ Urine is isotonic in the proximal convoluted tubule.
$(4)$ Urine becomes more and more hypotonic as it passes through Henle's loop.