WHAT IS THE TOPIC OF THE PASSAGE

1. - File thứ 1: De va dap an thi thu lan 2, Anh
Tiếng Anh File thứ 1: De va dap an thi thu lan 2, Anh
1. What is the topic of the passage? A. Aircraft design in the 20th century. B. The development of the helicopter. C. Which aircraft was the first to fly. D. The invention of the swash plate.