Please look at this question.
______ Ms. Santaro and her associate have proved to be able and honest in their dealing with clients.
(1)Between (2)Both

The correct answer is (1)Between. I've been really and so much surprised it should be "Between",not "Both".
Isn't "Between" a preposition ? Can preposition phrase be a subject in a sentence ? Furthermore, I think there seems to be a missing word like a noun if its correct answer is really (1)Between.
Could you explain to me why it should be (1)Between?
Thanks in advance.
Rest assured, both is the correct answer. If your source truly says that between is correct, then it’s time to find a new source.
Thank you so much for your confirmation,Mr.Aspara Gus. Emotion: embarrassed