German submarine U-21 (1936)
Summary of raiding history

Date
Name
Nationality
Tonnage
Fate
21 November 1939
HMS Belfast
Royal Navy
11,500
Damaged (mine)
1 December 1939
Mercator
Finland
4,620
Sunk
21 December 1939
Carl Henckel
Sweden
1,352
Sunk
21 December 1939
HMS Bayonet
Royal Navy
605
Sunk (mine)
21 December 1939
Mars
Sweden
1,475
Sunk
31 January 1940
Vidar
Denmark
1,353
Sunk
24 February 1940
Royal Archer
United Kingdom
2,266
Sunk (mine)

Question: Which Swedish ships did U-21 sink?
Answer: German submarine U-21 sank Mars and Carl Henckel both from Sweden on 21 December 1939.