9. if a farmer stops using his farmland for crops, and no other humans come to grow their crops
there, weeds and grasses will begin to grow. why is this considered secondary succession and not
primary succession?
a. a new ecosystem developed
b. there is already soil present
c. the land is not controlled by humans
d. wild grasses and weeds make soil from rocks

The answer is 'B'- There is already soil present.

Explanation:

One of the biggest difference between primary succession and secondary succession is that primary succession occurs where there is zero availability of  soil. In case of secondary succession, an already established community gets hampered due to man made activities or natural disasters like flood, rain, etc.

Since there is soil present in his farmland and vegetation is also available, it is the process of secondary succession.