To strengthen the introduction and provide context for the paper's subtopics, sentence B should be added:
B. Skilled trade positions are plentiful. Less plentiful are the men and women with the qualifications to fill them.
This sentence effectively sets up the discussion around the labor shortage in skilled trades, which aligns well with the paper's focus on the need for more trade schools to address this gap.
The other sentences, while relevant to the broader topic, do not directly enhance the introduction regarding the context of the worker shortage and the impact of trade schools on the job market.