Which of the following is a mixture

a. ice cube

b. candy bar

c. sugar

d. cotton ball

B.) Candy Bar because contains multiple ingredients.

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I think the answer is D.a cotton ball as cotton balls are made up of  fibres of 2 different materials :polyester, and cotton

all the other options are compounds

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.