Where do the electrons that enter the electron transport chain come from?

What best describes the dropping height of a ball that bounces back up to a height of 45 cm

During an experiment, readings for blood pressure in a persons body were found to be constant . however , when he measured by a different blood pressure cuff , the readings differed by 15 points for each reading. this difference indicates that the results are

_tissues respond quickly to outside stimuli a) epithelia b)nervous c)muscular d)muscular and nervous