7.49.txt

A)
Sales_data &combine(Sales_data);
// s converted to a temporary Sales_data object, nothing happens to i, because the constructor that initializes a Sales_data from a string, initializes units_sold and revenue to 0

B)
Sales_data &combine(Sales_data&);
// error: reference to temporary object

C)
Sales_data &combine(const Sales_data&) const;
// error: return type is Sales_data&, but the function is const: has a const Sales_data *this pointer, and returns it, therefore the return type should be const Sales_data&, or the function shouldn't be const