C++ MQCs with Explanation (CHAPTER 6: CONSTRUCTORS AND DESTRUCTORS)

Which one of the following is an implicit constructor for the class ROOM having a data member area?

1. Room ( ) { }
2. Room (int a) {area = a}
3. Room (Room & a) {area = a.area}
4. None of the above

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Answer: 

Option a: Room ( ) { }

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Explanation Here