The sunspot came into its active stage ,_____that happens on

The sunspot came into its active stage,____happens once every eleven years.
A.as
B.it
C.the one
D.what
我认为选B,为什么~

英语翻译
Afterwards he found a sunspot which lived long eonough to disappear from view on the westerner limb of the sun,to reappear on its easterner limb,and finally to regain its old position.
后来,他发现有一个停留在太阳西部的边缘上的黑子过了很长一段时间才消失在太阳的东部边缘上,最后再次获得了原位.

The sunspot came into its active stage ,_____that happens once eleven years..
The sunspot came into its active stage ,_____that happens once eleven years
A one B it C the one D what
为什么选A,不选其他?请详细回答.

英语翻译
On Earth,among the surest indications of sunspot cycles are believed to be the rate that trees grow,as seen in the rings visible in the cross sections of their trunks.(GWD题)
　　A.On Earth,among the surest indications of sunspot cycles are believed to be the rate that trees grow
　　B.On Earth,among the surest indications of sunspot cycles are,it is believed,the rate of tree growth
　　C.On Earth,the rate at which trees grow is believed to be among the surest indications of sunspot cycles
　　D.Among the surest indications on Earth of sunspot cycles,believed to be the tree growth rate
　　E.Among the surest indications on Earth of sunspot cycles is believed to be the rate at which trees grow
这个句子看不太懂,
另外,现在网上有的解释也看不甚明白,

The sunspot came into its active stage, __________ that happens once every eleven years. A．one B．it C．the one D．what

The sunspot came into its active stage last year, that happens once every eleven years. A．one B．it C．the one D．what