—______is your birthday gift, Linda?
— It's a circle.
A:What kind
B:What color
C:What time
D:What shape
氢原子的能级图如图所示,一群原来处于n=4能级的氢原子跃 迁到n=1能级的过程中( )
要配制200g 溶质质量分数为10%的氯化钠溶液,下列说法中正确的是 ()
甲、乙两个实心金属球,密度之比ρ甲∶ρ乙=2∶5,质量之比m甲∶m乙=1∶2,那么甲乙两球的体积之比V甲∶V乙为( )