There is only one parameter for an overloaded operator function (for a binary operator) when it is placed into a struct definition because:

A. there is only one return value
B. the left operand is not an object, so it must be passed into the parameter
C. if there were two parameters, the compiler would not know which one to use
D. the overloaded operator function is called for the operand on the left (which must be an object of a struct), so the right operand is passed in as a single parameter

D