Which of the following occurs to allow the kidneys to form concentrated urine?*1 pointA lot of NaCI is secreted at the ascending limb.A lot of NaCI is reabsorbed at the descending limb.A lot of water is reabsorbed at the descending limb.A lot of water is reabsorbed at the proximal convoluted tubule.

The kidneys form concentrated urine through a process that involves reabsorption of water and solutes at different parts of the nephron, which is the functional unit of the kidney.

1. A lot of NaCl is secreted at the ascending limb: This is partially correct. The ascending limb of the loop of Henle is impermeable to water but permeable to salts like NaCl. Therefore, NaCl is reabsorbed (not secreted) into the interstitial fluid, making the urine less concentrated.

2. A lot of NaCl is reabsorbed at the descending limb: This is incorrect. The descending limb of the loop of Henle is permeable to water but not to salts like NaCl. Therefore, water is reabsorbed (not NaCl), making the urine more concentrated.

3. A lot of water is reabsorbed at the descending limb: This is correct. As mentioned above, the descending limb of the loop of Henle is permeable to water but not to salts. Therefore, water is reabsorbed, making the urine more concentrated.

4. A lot of water is reabsorbed at the proximal convoluted tubule: This is also correct. The proximal convoluted tubule is the site of most reabsorption in the nephron. About 65% of the filtered water is reabsorbed here, along with solutes like NaCl, glucose, and amino acids.

So, the correct answer to the question "Which of the following occurs to allow the kidneys to form concentrated urine?" would be "A lot of water is reabsorbed at the descending limb" and "A lot of water is reabsorbed at the proximal convoluted tubule."