Crop rotation is important because:

some plants use a lot of nitrogen.
it increases soil fertility.
repeated planting of the same crops decreases soil quality.
all of the above.

Option D, All of the above

Explanation:

Crop rotation is the process of cultivating different varieties of crop on the same farm/land  based on the season. This is done in order to regulate the use  of  a particular nutrient from soil. When one crop (A)  is harvested a certain nutrient let’s say phosphorus get used,  now another crop (B) which uses nitrogen is grown, this allows the soil to replenish the lost phosphorus and make itself ready for crop A again in the next season.

Thus, crop rotation is necessary to obtain all the three option A, B and C.  

Thus , option D is correct.

it is a

explanation:

the answer is; comparing the frequency of collisions with and without the device

this makes the anti-collision system an independent variable and the collisions a dependent variable. if there are fewer collisions with an anti-collision system that without, then it can be stated that the system is a success. essentially, collisions under an anti-collision system are compared with the frequency of collisions by cars with the human drivers.