Answer:
D) A thing or event that causes a reaction.
Explanation:
Thermal energy to Gravitational PE to Kinetic energy.
Bacteriophages are viruses that infect and duplicate within a bacterium. Temperate phages (such as lambda phage) can keep reproducing using both the lytic and the lysogenic cycle.
SO.... Bacteriophages
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Ecosystems disturbed by a nature will be eventually restored unless it's a really big cataclysm, but most of the ecosystems are now a human caused disturbance. For example there are plenty of species which are already extinct or nearly extinct because of hunting industry.