This is a strengthen with a necessary premise question. We can use the negation test to test each answer choice. The correct answer, when negated, will wreck the argument. When the negation wrecks the argument, this shows that the answer choice in its original form is required for the argument to make any sense.
(B) says, "Fixed nitrogen is currently the only soil nutrient that must be supplied by artificial fertilizer for growing wheat crops."
(B)'s negation would say, "Fixed nitrogen is not currently the only soil nutrient that must be supplied by artificial fertilizer for growing wheat crops."
(B)'s negation doesn't wreck the argument because the original statement is so strong. It's true that (B) does strengthen the argument because if there are other nutrients contained in the fertilizer, then it's possible that even if we solve the nitrogen problem, we would have to keep on using the fertilizers. However, it's still not necessary to make this conclusion because it's still possible to reach the conclusion even if there were currently other nutrients in the fertilizer. It's possible that we'd be able to supply those nutrients in other types of ways in the future, and because the conclusion is based on the hypothetical of succeeding in producing Rhizobium in wheat plants, the future isn't off-limits. Thus, (B) can be eliminated from contention.
(E) says, "Rhizobium bacteria living in the roots of wheat would produce fixed nitrogen."
(E)'s negation would say, "Rhizobium bacteria living in the roots of wheat would not produce fixed nitrogen."
It's the fixed nitrogen that fertilizes the crops. If the bacteria don't produce this in the wheat crop (which is what the negation of (E) says,), then it will not fertilize and will not work. Thus, (E)'s negation wrecks the argument, so (E) is the correct answer choice and is a necessary premise.
Does this make sense? Let us know if you have any other questions!
