矿石性质对磨矿浓度的技术阐述正确的是（）

A、磨矿浓度较高时，介质受到的浮力大，打击力弱，但打击和研磨矿粒的几率增大

B、给矿粒度粗、硬度大、密度大的矿石，磨矿浓度应高一些

C、当磨矿浓度太高时，将大大降低介质的冲击力和研磨活动性，降低磨矿效率

D、当磨矿机产品粒度在0.15mm以下，或者磨碎密度小的矿石时，磨矿浓度通常控制在65～75%

---- Would you like to go travelling with me to New York, Benjamin?

---- Sorry, I’m busy, for the final examination is coming. So I ____________ hard throughout the next several weeks.

A．have worked

B．have been working

C．work

D．will be working