Is your question coming with a multiple choice answers? Or is it like free write? 
But here
Typically some elements that are used to convey a theme are the setting, plot, characters involved. You kinda have to question what the story/poem/article etc is about. Is it about [blank] or what is [blank] trying to tell us. 
Sorry if this doesn’t make sense lol I tried my best
        
             
        
        
        
'There is some evidence that using mouthwashes does have some clinical benefit in reducing levels of plaque - but only by a bit,' says Professor Damien Walmsley, scientific adviser to the British Dental Association. 
They are helpful, they are never going to be a replacement for the mechanical action of a toothbrush for getting teeth and gums clean.'
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The correct answer for this question is "D. Oh how little seems the spanMeasured round the life of man." Among the following choices, the line in the excerpt from the poem "A Grain of Sand" by Frances Ellen Watkins Harper that indicates its subject is that Oh how little seems the spanMeasured round the life of man.
        
                    
             
        
        
        
I would say no because a simile is having something referred to something else for example as brace as a lion or crazy like a fox.