In this question, it is important to remember that we have not been given any information about drops in trace elements other than selenium. So we cannot say that selenium is definitely the cause of the extinctions.
A – Wrong
We are not given any information about why marine life developed on Earth. We just know that without trace elements, we die but we do no know whether they helped to develop marine life.
B – Correct
This is correct as it does not use any strong words such as ‘must’ which is used in A. Also since, selenium is one of the trace elements, we can assume that a drop in its levels could contribute to the extinction.
C – Wrong
We cannot determine whether selenium is more crucial than other trace elements.
D – Wrong
‘Must’ is a very strong word to use as we are not given any information about the other trace elements.