牙石是（）
A.钙化的食物残渣
B.钙化的烟斑
C.钙化的脱落上皮细胞
D.钙化的菌斑
E.钙化的茶锈
参考答案：D
连词成句。
1. Jim that I lost the key found yesterday
2. I who is know the man a model worker
3. The I received yesterday letter that was from my parents
4. This who saved is the doctor the boy's life
一个2Ω的电阻接在电路中，若通过的电流为0.3A，通电时间为10min，则电流通过该电阻时产生的热量是______ J．
