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CAT Critical Reasoning Practice question with Solution 73

Rhizobium bacteria living in the roots of bean plants or other legumes produce fixed nitrogen which is one of the essential plant nutrients and which for non-legume crops, such as wheat, normally must be supplied by applications of nitrogen-based fertilizer. So if biotechnology succeeds in producing wheat strains whose roots will play host to Rhizobium bacteria, the need for artificial fertilizers will be reduced.

The argument above makes which one of the following assumptions?

[A]. Biotechnology should be directed toward producing plants that do not require artificial fertilizer.
[B]. Fixed nitrogen is currently the only soil nutrient that must be supplied by artificial fertilizer for growing wheat crops.
[C]. There are no naturally occurring strains of wheat or other grasses that have Rhizobium bacteria living in their roots.
[D]. Legumes are currently the only crops that produce their own supply of fixed nitrogen.
[E]. Rhizobium bacteria living in the roots of wheat would produce fixed nitrogen.
Answer: E

Fixed nitrogen is an essential nutrient that normally has to be supplied to non-legumes like wheat by means of fertilizers. However, Rhizobium bacteria living on legume roots produces fixed nitrogen. The author concludes that if we can develop wheat strains that also allow Rhizobium bacteria to live on their roots, the need for artificial fertilizers will decline. Why? The author must also believe (E), that Rhizobium growing on wheat will also produce fixed nitrogen (just as it does when it grows on legumes), and the wheat’s need for artificially-supplied fixed nitrogen will be reduced. But this is an assumption, since the evidence only says that Rhizobium growing on legumes produces nitrogen. If we negate or deny (E), and assert that Rhizobium would NOT produce fixed-nitrogen on wheat, we’d then have absolutely no reason to believe, as the conclusion proposes, that the need for fertilizers would be reduced.

(A) introduces the concept of what biotechnology ought to do, which is irrelevant; the conclusion only speaks of what will happen if Rhizobium-friendly wheat is produced.

(B) The conclusion merely says that the need for artificial fertilizers will be reduced, not eliminated entirely; it’s entirely possible that fertilizers will still be needed to provide other nutrients.

(C) needn’t be assumed; even if some strain of grass already has Rhizobium bacteria living in its roots, and even if that Rhizobium produces fixed nitrogen (which (C) neglects to say), the conclusion that the overall need for chemical fertilizers would be reduced by the production of further new strains of Rhizobium-friendly wheat wouldn’t thereby be invalidated.

(D) Nothing in the argument requires that no crops other than legumes produce their own fixed nitrogen; if there were rutabagas or tomatoes, for example, that also produced fixed nitrogen, it wouldn’t damage the conclusion that Rhizobium could reduce wheat’s need for artificial fertilizer.

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