Legitimate is a synonym of Natural. Natural is a synonym of Inevitable. Inevitable is the opposite of Possible. 
Hear is related to Noise. you are likely to find Noise in/at/on Classroom. you are likely to find Globe in/at/on Classroom. 
Legitimate <--Synonym--> Natural <--Synonym--> Inevitable <--Antonym--> Possible 
Hear <--RelatedTo--> Noise --AtLocation--> Classroom <--AtLocation-- Globe 