To enhance the introduction by providing context for the subtopics of the paper, the most appropriate sentences to add would be:
B. Skilled trade positions are plentiful. Less plentiful are the men and women with the qualifications to fill them.
This sentence effectively sets the stage for discussing the worker shortage in skilled trades, which is a central theme of the paper.
Additionally, I would suggest including:
D. Traditional vocational training included construction and mechanics. Today, trade schools train everyone from electronics engineers to pharmacy technicians.
This sentence provides a broad overview of the evolution of trade schools and the diverse opportunities they now offer, which is important for understanding the current context of the trade school debate.
Therefore, the best additions to the introduction would be sentences B and D.