Question: What unit was directed by Shiro Ishii and based at the Pingfang district of Harbin?
Context: Shuangcheng District () is one of nine districts of Harbin, the capital of Heilongjiang province, People's Republic of China, covering part of the southwestern suburbs.  It sits approximately 42 km south-southwest of downtown Harbin.  Formerly a county-level city until 15 May 2014.  The westernmost county-level division of Harbin City, it borders Daoli District to the north, Nangang and Pingfang Districts to the northeast, Acheng District to the east, and Wuchang to the southeast, as well as the Jilin prefecture-level divisions of Changchun to the south and Songyuan to the southwest. Nangang District () is one of nine districts of Harbin, the capital of Heilongjiang province, People's Republic of China, forming part of the city's urban core.  It is home to major offices of the provincial government and is the political heart of Heilongjiang province.  Other areas of interest within the district are the Harbin Railway Station, Guomao underground shopping street, a Confucian temple and the Buddhist Jile Temple.  By far the most populous and densely populated of Harbin's county-level divisions, it borders the districts of Daowai and Xiangfang to the northeast, Pingfang to the southeast, Shuangcheng to the south, and Daoli to the west.  A new subway system is also being built currently. Xiangfang District () is one of nine districts of Harbin, the capital of Heilongjiang province, China.  It is an urban district in the pre-1940 part of Harbin, bordering the districts of Daowai to the north, Acheng to the southeast, Pingfang to the southwest, and Nangang to the west.  It is an industrialized area, and contains the main scientific, technological and industrial zones of the city.  The Harbin Development Zone is in the district. Unit 731 (Japanese: 731部隊 , Hepburn: Nana-san-ichi Butai ) was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army that undertook lethal human experimentation during the Second Sino-Japanese War (1937–1945) of World War II.  It was responsible for some of the most notorious war crimes carried out by Japan.  Unit 731 was based at the Pingfang district of Harbin, the largest city in the Japanese puppet state of Manchukuo (now Northeast China). Pingfang Railway Station is a railway station of Lafa–Harbin Railway and located in the Pingfang District of Harbin, Heilongjiang province, China. Pingfang District () is one of nine districts of Harbin, the capital of Heilongjiang province, People's Republic of China, forming part of the city's urban core.  The least spacious of Harbin's county-level divisions, it borders the districts of Xiangfang to the north, Acheng to the east, Shuangcheng to the southwest, and Nangang to the west. The American cover-up of Japanese war crimes occurred after the end of World War II, when the occupying US government granted political immunity to military personnel who had engaged in human experimentation and other crimes against humanity, predominantly in mainland China.  The pardon of Japanese war criminals, among whom were Unit 731's commanding officers General Shiro Ishii and General Masaji Kitano, was overseen by General of the Army Douglas MacArthur in September 1945.  While a series of war tribunals and trials was organized, many of the high-ranking officials and doctors who devised and respectively performed the experiments were pardoned and never brought to justice.  As many as 12,000 people, most of them Chinese, died in Unit 731 alone and many more died in other facilities, such as Unit 100 and in field experiments throughout Manchuria. Harbin Aircraft Industry (Group) Co., Ltd. (HAIG), often shortened to Hafei (), is an aircraft manufacturing company headquartered in Pingfang District, Harbin, Heilongjiang province, China.  It was previously called Harbin Aircraft Manufacturing Corporation (HAMC) in English. Surgeon General Shirō Ishii (石井 四郎 , Ishii Shirō , ; June 25, 1892 – October 9, 1959) was a Japanese army medical officer, microbiologist and the director of Unit 731, a biological warfare unit of the Imperial Japanese Army involved in forced and frequently lethal human experimentation during the Second Sino-Japanese War (1937–1945). Harbin No.3 High School (Chinese: 哈尔滨市第三中学) is the most prestigious high school in Harbin, Heilongjiang Province.  And it is one of the top high schools in China.  It has two campuses.  One is located in the center of Harbin, Nangang District, and the other one is located in Qunli District of Harbin.  Harbin No.3 High School is a historical cultural site of Harbin due to its long history.  Qunli District campus is more than 240,000 meter squared with its entrance gate more than 50 meters wide.  It is one of the largest in China.The admission is given based on test results.  Among all the candidates in Harbin and surrounding cities, only the top 1% students will be selected.
Answer: Unit 731