travel can broaden your mind, I don't think it's a good idea to spend too much time and money on it .   [ ]

A. If    

B. While    

C. Because  

D. As  

首先发现电流周围存在磁场的科学家是（　　）

A．焦耳

B．法拉第

C．奥斯特

D．赫兹