与永磁体相比，电磁铁具有以下几个优点：
（1）电磁铁磁性的有无，可以由____ 来控制．
（2）电磁铁磁性的强弱可以由____ 来改变．
（3）电磁铁的N、S极是由____ 来决定．

   Thank you for joining CCTV’s Around The World show. Today, we're in Australia. It’s a beautiful,1day! There are many people here2vacation. Some are taking photos.3are lying on the beach. Look at this group of people playing beach volleyball. They look cool! I am surprised they can play4 this heat. This is a very interesting place. The people are really very5