21. In most cities, although there is a good public transport system, ______ private car is still becoming ______ popular means of transport.

A. /, the                   B. the, the               C. the, a                     D. a, /

22. Scientists have developed a new system ______ to give a warning seconds before an earthquake.

A. having been designed                            B. designed  

C. to design                                              D. designing