What is mean by old bonds broken
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When we make friends, we bond with them in various ways, but those bonds can be broken. When we no longer are friends with them, for whatever reason, we refer to it as " old bonds being broken "

Explanation:

Chemical reactions make and break the chemical bonds between molecules, resulting in new materials as the products of the chemical reaction. Breaking chemical bonds absorbs energy,while new bonds releases, energy with the overall chemical reaction being endothermic or exothermic.

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 correct answer would be d. hope this !