C++ MQCs with Explanation (CHAPTER 4: FUNCTIONS IN C++)

Following is a list of function declarations with default values. Which one of them is a valid declaration?

1. float mul(float a=5.0, b=10.0);
2. float mul(int a, float b=10.0, int c);
3. float mul(int a=5, float b);
4. float mul(int a, float b=10.0);

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Answer: 

Option a: float mul(float a=5.0, b=10.0);

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Explanation Here