The two most important regulatory systems for the maintenance of homeostasis are

Circulatory and nervous
Endocrine and circulatory
Lymphatic and circulatory
Nervous and endocrine

it has one origin of replication, but it proceeds in two directions - d)

the most common energy conversion is from kinetic energy to potential energy, and then from potential energy to kinetic energy.

later on comes the heat energy, mechanical energy, and other energy conversions.

this energy conversion is so common that we can find it in our day to day lives as well.

- one of the most common energy conversions involves the changing of potential energy to kinetic energy or kinetic energy to potential energy