To make a smooth transition from the first paragraph to the second paragraph, it is important to use transition words or phrases. These words help to connect ideas and make the text more coherent and readable. Some examples of transition words that could be used in this context include “However”, “But”, “On the other hand”, “In contrast”, and “Nevertheless” .
Therefore, option D would be the most appropriate sentence to add at the end of the first paragraph to make a smooth transition to the ideas in the second paragraph.