Agriculture was a step in human progress （） which subsepuently was not anything comparable until our own machine age.

单项选择题Experiments in the photography of moving objects （） in both the united states and Europe before 1990.

A．have been conducting
B．were conducting
C．had been conducted
D．are conducted

单项选择题Speech is the （） ability possessed only by human beings.

A．unique
B．average
C．collective
D．single