Biologists working in Spain say that their discovery of teeming life in a highly acidic river may
not only broaden the search for life, or for evidence of past life, no other planets but also show
that a number of forms of microscopic life can adapt to conditions that scientists have long
thought hostile to all but the hardiest bacteria.
A. show that a number of forms of microscopic life can adapt to conditions that scientists have long
thought hostile to all but the hardiest bacteria
B. may show that a number of forms of microscopic life is capable of adapting to conditions that
scientists have long thought hostile to all bacteria but the hardiest ones
C. shows a number of forms of microscopic life to be capable to adapt to conditions that scientists
have long thought had been hostile to all but the hardiest bacteria
D. showing that a number of forms of microscopic life is capable of adapting to conditions that
scientists have long thought had been hostile to all but the hardiest bacteria
E. showing that a number of forms of microscopic life can adapt to conditions that scientists have
long thought hostile to all bacteria but the hardiest

OA: A

Could anyone let me know why the verb "broaden" is used? According to me it should be "broadens".
But since it is an OG question, by default it has to be correct. What is the reasoning behind using "broaden"?