the famous scientist albert einstein sometimes used "thought experiments" such as imagining himself chasing a beam of light or riding in an elevator in free-fall. these thought experiments, which would not be possible to conduct physically, helped him challenge current specific thinking and develop his the worked of gravity and of special relativity

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

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.