Which of the statements is correct? A. Water leaves the descending limb of the loop of Henle through active transport. B. NaCl enters the ascending limb of the loop of Henle through diffusion. C. NaCl leaves the descending limb of the loop of Henle through active transport, and water enters the ascending limb of the loop of Henle through osmosis. D. Water enters the descending limb of the loop of Henle through osmosis. E. NaCl leaves the ascending limb of the loop of Henle through active transport.

The best answer is "E"

NaCl leaves the ascending limb of the loop of Henle through active transport.

Explanation:

The loop of Henle is a U-shaped portion of the tubule that conducts urine within each nephron. It's principal function is the recovery of water and sodium chloride from the urine.

The ascending limb is impermeable to water. NaCl is pumped from the tubule into the interstitium in the ascending limb by active transport. This active transport enables the kidney to establish an osmotic gradient which enables the permeability of water and urea thereby allowing the kidneys to concentrate the urine.

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1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.