The image below is a model showing how chromosomes and DNA are structured within a cell. W
X
Y
Z

Which letter represents a gene?

Z is gene.

Explanation:

W is nucleus

X is chromosomes

Y is DNA

the fossil record provides evidence of a common ancestor to many species.

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.