Which of the following elements do NOT have an oxidized form that can serve as an electron acceptor in microbial redox reactions in natural systems? Select all that apply. There could be more than one.
A. manganese (Mn)
B. magnesium (Mg)
C. calcium (Ca)
D. nitrogen (N)
E. carbon (C)
F. iron (Fe)
G. sulfur (S)
