——David, could you tell me _________?——Every four years.
A:when the 2014 World Cup will finish
B:how often the World Cup takes place
C:how many teams take part in the World Cup
D:who may score the most goals in the World Cup
某粒子含有10个电子,10个中子,电荷为2﹣,则它的化学符号是( )
按照玻尔的原子理论,大量氢原子从n=5的激发态向低能级跃迁时,最多能向外辐射( )
下列关于生物类群及主要特征连线错误的是( )