龈上牙石是

A．不能直接观察到牙石

B．质地坚硬的牙石

C．附着较紧，较难去除的牙石

D．沉积于牙周袋内的牙石

E．沉积于龈缘以上牙面的牙石

--  do you improve your English ?

--I improve it    practicing it more often.

A．How; with

B．What; with

C．How; by

D．What; by