By Prepositions, TOEFL

Hello Everyone! I took the following tests on TestMagic. Though TestMagic provided very good explanations for their tests but I had problem with the following samples, the explanations were very simple. I was wondering if someone could better explain it for me... 8. Uranium, ______ , is an...

In today's GetTOEFL daily quiz (Mon Jan 10), the correct answer for the preposition to be filled with following the noun 'doubt' was 'over'. But isn't it correct that you use 'about' also?