How do scientists know when a mass extinction took place? A. they find more fossils of plants B. they find fossils of animals that died suddenly C. they find a sudden disappearance of many fossil species D. they find evidence of changes in Earth's atmosphere

2kg of refrigerant 134a undergoes a polytropic process in a piston-cylinder assembly from an initial state of saturated vapor at 2 bar to a final state of 12 bar, 80 degree c. a)determine the work for the process in kj. b)sketch the process on a p-v diagram.

Question 11 of 13 (1 point) jump to question: a stairway, ladder, or ramp must be present in excavations in which of the following situations? a. all trenches must have access/egress b. the trench is more than 15 feet wide but only 1 ½ feet deep c. the trench is more than 4 feet deep and the devices must be within 25 feet of all workers

In a hydrogen molecule there are a total of four charges, 2 protons in the two nuclei, and 2 electrons. how many unique charge-pairs are there (without double counting)?