Which do the following is an impact of growing the same crop on a farm for several years?
a. improved soil quality
b. decrease in plant disease
c. increase in diversity of plants
d. decrease in the fertility of soil

The correct answer is D. Decrease in soil fertility.

A particular crop has a particular need for certain nutrients, and contributes a certain set of nutrients in return to the soil. When the same crop is planted over and over the years, the certain set of nutrients required to sustain are used up, and an accumulation of particular nutrients not essential for that particular plant's growth piles up. This leads to poor growth and development of the plant due to nutrient deficiency - and the quality of a soil to sustain growth and agriculture of a plant is known as fertility, which in this case decreases.

i think the miocene epoch spans the time from 23 million to 12 million years ago.