In the past few months, the country _____ a further tightening of supply restrictions, making fuel and food supplies more difficult to obtain.

A．would witness

B．witnesses

C．has witnessed

D．had witnessed

若一气泡从湖底上升到湖面的过程中温度保持不变，则在此过程中关于气泡中的气体，下列说法中正确的是[ ]

A．气体分子间的作用力增大

B．气体分子的平均速率增大

C．气体分子的平均动能减小

D．气体组成的系统的熵增加