"the use of weed killers has removed a limiting factor to the size of populations of crop plants."

True

Explanation:

Question -

"The use of weed killers has removed a limiting factor to the size of populations of crop plants."

True

Solution -

Weeds are basically unwanted plant species that grow in between the main crop thereby hindering the potential growth of main crops. As the time required by the main crop plant to mature is larger than the time required by the weed plants. Hence, with in a short span of time weed plants multiply thereby consuming essential mineral and nutrients from soil and fertilizers. They also cover the physical area thereby restricting the expansion and development of main crops. Application of weed killer will kill the weeds and remove the limitation hindering the growth of main plant/crop

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it is either c or d and i know for sure it is not a or b