在固定成本较高的行业中，经营杠杆及其产生的信用风险也较高。( )
参考答案：对
He _____ articles for our wall-newspaper these three years, and he _____ about forty articles.
A．has been writing； has written
B．has been writing； wrote
C．is writing； has been writing
D．has written； has written
试求如图所示电路中A点的点位（）
A.20V
B.10V
C.-8V
D.8V
