试验线圈的感抗可用哪一个公式计算？（）
A.XL=2πL
B.XL=1/2πL
C.XL=2πf·L
D.XL=IR
参考答案：C
______ that he has no qualifications in business management, Ken plans to gain the necessary skills by taking a part-time course.
A．Concerning
B．Concerned
C．Being concerned
D．To concern
空燃比
