问题描述：
[单选]
来自一颗5等星的光子每秒落在冥王星表面1cm2上的数量为10000个,那么如果使用口径6米的BTA望远镜,在地面上每半小时能够接收到来自一颗20等恒星的多少个光子 On 1 cm2 of Pluto"s surface fall approximately 10,000 photons per second from a star of the fifth magnitude. How many photons would fall on a detector from a star of 20m during half an hour, if BTA at the Earth is used (the diameter of the main mirror is 6m)
A．5.09 * 10^6(10的6次方)个光子
B．1000000个光子
C．40000个光子
D．56 * 10^6(10的6次方)个光子
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