Which of the following is one reason that unions play a decreased role in the US economy today? A.
Many large US factories have closed as companies seek lower-priced labor elsewhere.
B.
Over time, the collective bargaining process has helped define acceptable actions for management and labor.
C.
Long-time union efforts have led to higher wages and improved working conditions.
D.
Regulations established by the US government have limited the effectiveness of unions.

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 population of unionid mussels in the hudson river decreased after zebra mussels were introduced. zebra mussels, a nonnative species, competed with the unionid mussels for food. the zebra mussels were able to outcompete the unionid mussels, allowing the zebra mussel population to increase dramatically over six years. the unionid mussel population decreased due to a lack of food.

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