In ancient times, some catastrophic extinction of species occurred ______, but such catastrophes were comparatively rare.

A．about 30 New York women who offered free advice by day
B．three women freelancers about 30 years old who offered advertising advice on Saturday
C．about 30 women advertising freelancers offered advice every Saturday afternoon in New York
D．three women about 30 years old, who did advertising as a job, offered free advice every Saturday afternoon

A．The new commercial bike has no crossbar and its centre of gravity is lower than the Olympic model.
B．When the rider is doing some rough riding, the new bike’s low centre of gravity gives him better control.
C．The new bike is made by using aerospace technology and is quicker than the conventional bike by 3 seconds per kilometre.
D．The new bike has no metal bar between the saddle and the back wheel, and the amount of carbon fibre used in the back wheel can be changed according to the user’s tast