A：中子数
B：电子数
C：中子数与电子数之和
D：中子数和电子数之差
在消化道的某一部位内抽取内容物进行化验，结果发现其中的脂肪类物质还没有被消化，蛋白质却已被初步消化，那么该部位应是____．
It is fairly common in Africa for there______ a group of expert musicians surrounded by others who also join in the performances.
