As is known to all language learners, the newly learnt words will soon be forgotten unless ______ used in everyday communication.

A．gradually

B．selflessly

C．fluently

D．frequently

如图所示实验说明：___________________________，这种现象叫做电流的磁效应，这是由_________首先发现的，从而第一个发现了电与磁之间的联系。