ਪੈੱਗ ਪੈੱਗ ਕਰਦੇ ਨੇ ਬੋਤਲ ਮੈਂ ਚਾੜੀ
ਤੂੰ ਕੀ ਸਾਨੂੰ ਛੱਡਣਾ ਨੀ ਅਸੀਂ ਛੱਡੀ ਯਾਰੀ.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

the answer is c.the valence of the atoms combine .

c) should be the answer.