Which statement best describes the role that gravity played in the formation of our solar system? a. gravity allowed the nebula to expand and move outward. b. gravity caused the nebula to cool enough for planets to form. c. gravity removed excess gas and dust from the cores of the planets. d. gravity pulled particles of dust and gas together to form planets.

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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