user: Question: When Marja Ridder won the Women's singles who won the Men's singles?
Columns: ['Year', "Men's singles", "Women's singles", "Men's doubles", "Women's doubles", 'Mixed doubles']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['1958', 'Knud Aage Nielsen', 'Tonny Holst Christensen', 'Hugh Findlay Ronald Lockwood', 'Heather Ward Barbara Carpenter', 'Hugh T. Findlay Heather Ward']
SQL:

assistant: SELECT "Men's singles" FROM "Table" WHERE "Women's singles" = 'Marja Ridder';