Alright so let's break this passage down. A group of scientists discovered that removing the rats' parathyroid glands resulted in the rats having substantially lower than normal levels of calcium in their blood. They hypothesized from this that the function of the parathyroid gland is to regulate the levels of calcium in the blood by raising that level when it falls below the normal range.
In a subsequent experiment, the scientists removed not only the parathyroid gland, but also the adrenal gland. They discovered that the level of calcium in the rats' blood decreased much less sharply than when the parathyroid gland alone was removed.
So let's place numbers here so we can better visualize. Let's say the normal level of calcium in rats' blood is 10. Removing just the parathyroid glands resulted in the blood calcium level to drop to 3. Whereas removing the parathyroid gland and adrenal gland resulted in the blood calcium level to drop to 6.
Answer choice (C) states: "The absence of a parathyroid gland causes the adrenal gland to increase the level of calcium in the blood."
This answer choice does not explain why removing only the parathyroid gland caused the calcium levels to drop drastically whereas removing both adrenal and parathyroid slowed the decrease. Answer choice (C) only tells us that the adrenal gland works to raise the calcium levels when the parathyroid gland is removed.
We still do not know why when both are removed the decrease slows. Technically, if the presence of the adrenal gland helped add calcium, then removal of the adrenal gland should have shown a steeper decrease in the calcium levels (since another gland that was adding calcium to the blood was removed), as opposed to the level of calcium decreasing "much less sharply."
Answer choice (A) states: "The adrenal gland acts to lower the level of calcium in the blood."
So, if the scientists' hypothesis is correct, i.e. the function of the parathyroid gland is to raise calcium levels when it falls below normal range, then it would make sense that if only the parathyroid gland was removed--leaving the calcium-lowering adrenal gland--the calcium levels would drop drastically, whereas if both the parathyroid gland and adrenal gland were removed, then the adrenal gland wouldn't be present to lower the already unregulated calcium levels, which would result in a less sharp decrease of the calcium.
Thus, answer choice (A) explains the surprising discovery in a way that is consistent with the scientists' hypothesis.
Hope that clears things up! Please let us know if you have any other questions.
alliehall21June 10, 2020
I am still not understanding the explanation for A
SkylarJune 14, 2020
@alliehall21, happy to help!
The passage tells us: - Scientists found that removing the PG resulted in lower levels of calcium - So, the scientists hypothesized that the PG functions to raise calcium levels as needed - Scientists found that removing the PG alone results in a larger calcium drop than does removing both the PG and AG, which is a surprising discovery
We are asked to explain this last point made in the passage in a way that upholds the scientist's hypothesis that the PG functions to raise calcium levels when necessary.
Let's say we have two laboratory rats: (1) Lab Rat A has both his PG and AG removed - higher calcium levels (2) Lab Rat B has only his PG removed - lower calcium levels
According to the passage, Lab Rat A has higher levels of calcium than Lab Rat B does. Why would this be? We know that the PG can raise calcium, so removing it clearly results in lower levels. However, both rats have their PG removed. The only difference between the two is their AG.
If AG raised levels of calcium like PG, we would expect that removing it would cause lower calcium levels. This is the opposite effect of what we're seeing. So, what if AG functioned to lower levels of calcium instead? Then removing the AG would cause higher levels of calcium because the lab rat would no longer have glands functioning to decrease calcium. This is what (A) says, so it is correct.
Does that make sense? Hope it helps! Please let us know if you have any other questions!
