Here we have a strengthen with necessary premise question. Remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it does strengthen, we negate the answer choice to see if its negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.
Conclusion: "if biotechnology succeeds in producing wheat strains whose roots will play host to Rhizobium bacteria, the need for artificial fertilizers will be reduced."
Why? We are told that Rhizobium bacteria that live in the roots of bean plants or other legumes produce fixed nitrogen. We know that fixed nitrogen is one of the essential plant nutrients and that for non-legume crops, such as wheat, it is normally supplied by applications of nitrogen-based fertilizer.
What's the issue here? We only know how the Rhizobium bacteria reacts when it lives in the roots of bean plants or other legumes. We do not know how it will react when it is in the root of a non-legume, such as wheat. It could be possible that something about living in a non-legume prohibits the bacteria from producing nitrogen.
Answer choice (E) helps us get passed this exact issue, "Rhizobium bacteria living in the roots of wheat would produce fixed nitrogen." So, answer choice (E) strengthens the argument by giving us information on how the bacteria would react in the specific plant we are discussing.
Negation: Rhizobium bacteria living in the roots of wheat would not necessarily produce fixed nitrogen.
Does this make the argument fall apart? Yes.
If the bacteria does not produce nitrogen when living in a wheat plant, then it is no longer true that placing the bacteria in what roots would reduce the need for artificial fertilizers.
Hope that clears things up! Please let us know if you have any other questions.
