I Believe I Can Fly is a nice song by R. Kelly. This song tells us that believing in          is very important.

A. themselves  B. ourselves     C. itself

两个完全相同的电热器，分别通过如图所示的两个电压最大值相等的方波交流电和正弦交流电，则这两个电热器的热功率之比为____.