My school isn't far from here. It's only _____ walk.
A:fifteen minutes
B:fifteen minutes'
C:fifteen minute's
D:fifteen minutes's
小明说:“我的爸爸妈妈会给我很多零花钱,所以不需要节俭。” ( )
A、B是两个完全相同的电热器,A通以图甲的方波交变电流,B通以图乙所示的正弦交变电流,则两电热器的电功率PA:PB等于( )