He is very popular among his students as he always tries to make them_________ in his lessons.   [ ]

A. interested    

B.interesting  

C. interest    

D.to interest

焊条电弧焊时，电弧电压取决于弧长，弧长越长，则电弧电压（）。

A.越低

B.越高

C.不变

D.无规则变化