The correct option is a. "increased cortical mass, increased numbers of neurons, increased synaptic connections, increased neurotransmitter amounts, and increased myelination.
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Explanation:
Rodents that have been raised in enriched environments were found to have thicker cerebral cortex containing approximately 25% more synapses in a study conducted by Mark Richard Rosenzweig. This increase in <em>synapses</em> implies in an increase in <em>neurons</em> and <em>neurotransmitters</em>, which also indicates an increase in <em>myelination</em>.
Soils develop different horizons due to the amount of weather that is being exposed to each individual layer. What separates one horizon from another is the properties that make up each layer.