Unlike other amines `(RNH_(2))`, guanidine, `H_(2)overset(..)N-underset(..)underset(NH)underset(||)C-overset(..)NH_(2)`
is a strong base because the cation formed by the addition of `H^(+)` has `"________"` equivalent contributing structures.
A. three
B. four
C. two
D. five