1. What do epidemiologists study? A. risk factors for different diseases

B. how often a disease occurs in a particular area or group of people

C. blood and heart diseases

D. Non-infectious diseases and their causes

i am pretty sure the answer is c

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.