The correct answer should be additionally.
The word additionally would provide the best transition between these two sentence. It is so because you have one idea in the first sentence, and then you want to show another idea very similar to the first one in the second sentence. Using additionally to convey that is the way to go.
I believe the best answer here is A. it uses imperfect coordinating structure.
Explanation:
The quality or state of being pertinacious.
There is power in collective action as the one mouse could do nothing to the cat. But the one cat could do nothing to the dog, but 34 mice could because there is power in collective action.
