Correct answer:
A. Humans first began planting crops during the Neolithic Revolution. In addition, they also began domesticating animals.
I've underlined the transition words in that sentence -- "in addition." Words such as in addition, also, or likewise show a relationship of similarity to preceding material in the previous sentence. Transition words like but, rather, or, and however show a relationship of difference to what was stated in a prior sentence or paragraph. There are also transitional phrases that effects or consequences, like as a result or in consequence. There are transitional words/phrases that point to examples or support of a point: for instance, namely, as an illustration, etc. And there are transition words that set up a conclusion: as has been shown, in summary, etc.