白陶原料中含有瓷胎主要成分是（）
A、瓷石
B、高岭土
C、木节土
D、蒙脱土
参考答案：B
—Is Bob doing homework?
—No, he ______. He is ______ a letter. [ ]
A. doesn't,write
B. isn't, writes
C. isn't, writing
D. isn't, writeing
三极管两个（）构成。
