 Complete the following phrase:  The Presidential Building, also known as Zichao House and the Concrete House, was one of the most important buildings in the complex. Situated on the northernmost part in the palace, the six-storey house with the structure of reinforced concrete was one of the tallest buildings in Nanking in the 1930s. The construction started in 1934, and finished in December 1935. Lin Sen used this house as the office of Chairman of the Nationalist Government until the 