I believe A and B are the ones. As for explaining? hmm. I would honestly say that any number to the right of a number is a tenth of the place above it. But then again, it's optional to explain ;P
84% of 50 is 42. So if your test was graded out of 50 you would've gotten 42/50. If you multiply 50 by 2 (50x2), you have 100, so then you'd just divide the 84% by 2 so you'd get 42%
The answer is <u><em>0.76075</em></u>!
<em>Hope this helps!</em>