33 Which of the following is NOT true of the scramjet engine?
A It has been used on passenger planes．
B The air it breathes does not put out the combustion flames．
C It works at speeds greater than five times the speed of sounD.
D It goes as fast as a rocket．

单项选择题34 What does Werner J．A.Dahm say about the Hyper-X test flight?A It was unsuccessful because it lasted only 11 seconds．B It is a very important event in developing very fast airplanes．C，It allows people to fly at an altitude of 100,000 feet．D It can transport cargo quickly and cheaply to outer spacE.

多项选择题明代家具出现的社会背景是（ ） A、经济的恢复和发展 B、大兴住宅和建筑 C、海外贸易的发达 D、专门书籍的出现