Answer:
B: the shoot apical meristem
Explanation:
The test run during the research is meant to determine the approximate age of the treated element. During photsynthesis the plant incorporates the raidoactive carbon isotope, 14C. When the plant dies, these atoms start to transform in 14N (nitrogen isotope) due to radioactive decay. So, while the plant is alive it continues to store more and more 14C in time because of photosynthesis. This means that the more 14C atoms found, the older the part of the plant. The question asks where are you <em>least</em> likely to find 14C, so, we have to determine which part of the plant contains the youngest cells in order to find the answer.
Opcion "c" says the roots. Considering that the roots are the first part of the plant to form, then it shouldn't be the youngest part.
Option "d" suggests a mature upper leaf on the opposite side of the plant from the treated leaf. The fact that it mentions the opposite side of the plant doesn't give us any information that we could use. It is useful, though, to know that it is an upper leaf. This means that it grew after the one that's being treated, so it's youger and surely we would find less 14C atoms than on the treated leaf. Then, we already know that option "a" can't be the answer because the treated leaf came from a lower shoot, meaning it's older.
Here we have two options left. We have to compare "d" (mature upper leaf) with "b" (the shoot apical meristem). If we analyse this we could think that the upper leaf is younger than any cell at the shoot, since the shoot grew first, but, option "b" doesn't talk about any shoot cell, it makes reference to the apical meristem which is the one that takes care of growth. The apical meristem can be found at the tip of the plant, constantly regenerating at a high rate. So, this means that we are most likely to find the youngest cells here, therefore, we are <em>least</em> likely to detect 14C after the experiment. This is why the answer is: <u>B) the shoot apical meristem</u>.
