All people who came into contact with the H1N1 flu patient were isolated. All those who were isolated had contact with Xu Haihua. Assuming the above proposition is true, which of the following propositions is also true? ? ['Some people may not have been exposed to H1N1 influenza patients, but they have been exposed to Xu Haihua.', 'Xu Haihua is a patient with H1N1 influenza A.', 'Everyone who has been in contact with Xu Haihua is isolated.', 'All patients with H1N1 influenza have been in contact with Xu Haihua.']