将造成需求获取困难的问题是（）
A.预算
B.范围
C.理解
D.波动性
参考答案：B, C, D
The book __ __by the young writer sells well. It is popular among middle school students.
A．was written
B．writing
C．wrote
D．written
MC43气动调节器最常见的故障有哪些？
