Here are 2 Sentences from MGMAT
1) Since 1989, after the Berlin Wall had been demolished, one of the most problematic ethnic groups in the reunified Germany, in cultural and economic assimilation terms, were the former East Germans, who have had to acclimate to an entirely different political system.
2) A higher interest rate is only one of the factors, albeit an important one, that keeps the housing market from spiraling out of control, like it did earlier in the decade.
In the 1st sentence "one of the most problematic ethnic groups" is considered singular.
In the 2nd sentence "one of the features" is considered plural.
Further, I was researching how to identify Singular/plural and found another sentence :
3) Only one of the students was registerd.
In the 3rd sentence "one of the students" is considered singular.
Please explain how to identify Singular/Plural in "One of the" constructs.
Thanks.