Question

besides transfer due to machinery, what is another way that humans can introduce weeds while planting a crop?
through bird action
through rainfall
though wind action
through weed seeds mixed with the crop seed to be planted

Explanation:

The question asks for a human - related way (excluding machinery) to introduce weeds during crop planting. Bird action, rainfall, and wind action are natural processes. However, weed seeds mixed with crop seeds is a human - related factor as humans handle and plant the seeds.

Answer:

through weed seeds mixed with the crop seed to be planted