The correct answer is A. Only
Explanation
It is known as a modifier to words or phrases that act by modifying the core of the subject in a sentence, complementing its meaning. Among the modifiers of the subject's nucleus, there are the articles and adjectives that accompany the subject's nucleus noun, define it, and agree with it in gender and number. These modifiers are called direct modifiers. Therefore, Only is a modifier because it modifies the complete meaning of the phrase by modifying the core noun of the subject "they". So the correct answer is A. Only.
I think is first that what i think
It should not be controversial: it would only make the research less reliable since people would have their opinions about the topic that they would want to defend. It should also not be obscure: then it's hard to check.
The best answer is that it should be multifaceted: it should include a number of aspects, not just one, so that the test can check for those aspects, not just one.
