钻石是（）
A.有六方晶系2H型的多型变体称为六方金刚石
B.可以是水热法合成的
C.自然元素矿物
参考答案：C
______ over and over again, but he still couldn’t understand.
A．Having explained
B．Having been explained
C．Explaining
D．It had been explained
语音教学节目具有（）特点？
