公路全长s千米，某人步行t小时可达到，为了提前半小时到达。步行每小时应多走（ ）千米。
NASA has made a ________ that our earth has no threat associated with the film 2012 which is based on pure myths.
A．clarification
B．classification
C．identification
D．presentation
随着压力的增高，流量计的漏流量会（）
A.增加
B.减小
C.不变
D.为零
