Can we chat im really bored

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.

What is the missing step in this proof? scroll down to see the entire proof.u

Which of the following is a presentation cue? a. crossing your arms b. mumbling c. repeating words d. whistling