Which statement best describes a keystone species
a) it has little genetic variation
b) it has an extremely high population
c) it remains unaffected by all environmental changes
d) it has a strong influence on the ecosystem
e) it's the first species to populate a new ecosystem

Option D,  it has a strong influence on the ecosystem

Explanation:

Keystone species are an essential part of an ecosystem and their functions/mark cannot be replaced by any other organism. Once the keystone species is removed from an ecosystem, the ecosystem struggles to sustain  or it changes radically. A keystone species in majority of ecosystems/food web is a predator but this is not true in all ecosystems/or with all keystone species.  

For example – there was a trophic cascade when the Gray wolves in the greater Yellowstone ecosystem were removed.  

i believe the answer is a or 1

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.