It would be D, a MRI scan does not use radiation.
I would check this website to give you a little bit of a better idea on your question.
https://www.nytimes.com/2010/09/12/jobs/12search.html
let me know if this helps you at all.
Because smaller fish eat plankton. And then the bigger fish eat the smaller fish. Its really only important food source to help feed smaller fish. Hope this helps! Have a great day!