“上品无寒门、下品无士族”反映了隋唐时期的等级制度。
参考答案：对
PC和8254的0号计数器每隔 ______ ms产生一次时钟中断请求。
Everyone should learn something about first aid, because every second ____ in an emergency. [ ]
A. urges
B. costs
C. needs
D. counts
