Data Structures using C++

Here is a function prototype and some possible function calls:

double sumOfThreeNumbers(double = 1.0, double = 2.0, double = 3.0);

// Possible function calls:

cout << sumOfThreeNumbers ();

cout << sumOfThreeNumbers (2.3);

cout << sumOfThreeNumbers ( , 1.7, -3);

cout << sumOfThreeNumbers (2.3, , -3);

How many of the above function calls are legal?

A. 1 of them is legal.

B. 2 of them are legal.

C. 3 of them are legal.

D. All of them are legal.