Indeed,thanks to the good ideas of lots of people,like the cycling fans in the 1960s,many people around the world______the city centre streets without cars for many years.

A．have enjoyed

B．enjoyed

C．have been enjoying

D．have been enjoyed

锥子的尖端越尖，越容易扎进物体里，这是因为在压力一定的情况下，减小了______，增大了______的缘故．