English is difficult________. Nobody can learn to use it in a few minutes.

A．learning

B．to be learned

C．learned

D．to learn

下列关于药材浸提的叙述，正确的是

A．浸提温度越高越好

B．浸提时间越长越好

C．溶剂的pH越高越好

D．浓度梯度越大越好

E．浸提压力越高越好