Answer: A. Attribution (CC BY)
Explanation: because it allows the user to do whatever they like, but they have to give credit to the author of the product in order to use it.
The correct sentence is: Two of the best singers were them. This makes sense because when you rephrase the sentence, "them" becomes "they" which appropriately describes the nouns of the sentence. The modified sentence then becomes: They were the best singers.
