user: Question: Which Men's u20 has a Men's 45 of N/A?
Columns: ['Year', "Men's Open", "Women's Open", 'Mixed Open', "Men's u20", "Women's u20", 'Senior Mixed', "Men's 30", "Women's 30", "Men's 35", "Women's 35", "Men's 40", "Women's 40", "Men's 45", "Men's 50"]
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1997.0, 'Sydney Scorpions def Brisbane City Cobras', 'Brisbane City Cobras def Sydney Mets', 'Sydney Rebels def Brisbane City Cobras', 'Brisbane City Cobras def Gold Coast Sharks', 'Sunwest Razorbacks def Sydney Rebels', 'N/A', 'N/A', 'N/A', 'N/A', 'N/A', 'N/A', 'N/A', 'N/A', 'N/A']
SQL:

assistant: SELECT "Men's u20" FROM "Table" WHERE "Men's 45" = 'N/A';