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Which two statements are true about a system? 1. a system is a group of objects analyzed as one unit. 2. energy that moves across system boundaries is conserved. 3. there’s only one way to define the boundaries of a system. 4. all systems are made by humans.

Did the universe cool off or get hotter as it expanded after the 'big bang"?

Which of the following always exist between two objects having mass ● elastic force ● contact force ●gravitational force ●magnetic force