我国在唐代贞观年间发明了（） ["雕版印刷技术","泥活字印刷技术","木活字印刷技术","铜活字印刷技术"]
Cisco collaboration applications and services can be hosted in A number of ways, which three are valid hosting models?（） ["device-specific","on-premises","integrated","hybrid","virtual","SaaS"]
过时效
方差在任何情况下都是正数，协方差值可正可负。
摩擦学面临挑战
根据不通的签名过程，多重数字签名方案可分两类：即（）和（）。
