Answers !
scientists have genetically engineered crops to be resistant to herbicides. this allows farmers to kill weeds with small doses of herbicides that don’t harm the crops. why, then, are people concerned that transgenic plants may lead to an increase in the use of herbicides?
cross-pollination could cause the crops to lose their herbicide resistance.
weeds may naturally develop herbicide resistance with each dose.
herbicide-resistant crops may grow out of control and become weeds.
cross-pollination could pass herbicide-resistant genes to the weeds.

The correct option is Option C i.e. Herbicide-resistant crops may grow out of control and become weeds.  

Explanation:

1. The herbicide resistance is a genetic character that is introduced into the plant with help of genetic engineering.

2. Now crops are not sexually reproduced. New seeds are planted every season. So chance of losing the herbicide resistance is less.

3. The risk of weeds becoming herbicide resistant is there even with genetic engineered crops.

4. Cross-pollination between two different species doesn’t occur generally.

5. But if these plants are not killed by any herbicides, they themselves can grow too fast as weeds.  

A. Cross-pollination could cause the crops to lose their herbicide resistance.

Explanation:

Herbicide resistance is the acquired capacity of plants to endure and imitate following introduction to a portion of herbicide typically deadly to the wild kind. Herbicides don't actuate opposition in weed species, rather they basically select for resistant individuals that normally happen inside the weed populace.

Herbicide resistance, communicated by a recessive allele, advances and spread generally gradually as the dominant heterozygotes and homozygous prevailing phenotypes are dispensed with by use of the herbicide to which resistance created.

This is truthful for cross-pollinating species, be that as it may, in exceptionally self-pollinating species, the resistant alleles can spread rapidly as selfing increments the recurrence of homozygotes to the detriment of heterozygotes.

the answer is:

carbon dioxide