Science has told us so much about the moon that it is quite easy to form an idea of what it would be like to go there. It is certainly not a friendly place. As there is no air or water, there can be no life of any kind. Mile after mile there are only plains(平原) of dust with mountains around them. Above, the sun and stars shine in the black sky. If you step out of the mountains shadows, it will mean moving from terrible cold into great heat. The moon is also a very silent(安静的) world, for sound waves (声波)can only travel through air. But from the moon, you can see a friendly sight(景色). Our earth is shining more brightly than the stars. It looks like a large ball, coloured blue and green and brown.

①核糖体    ②线粒体    ③内质网    ④高尔基体．

已知函数f（x）=|x+a|﹣2a，其中a∈R．

已知函数$f\left(x\right)={\log }_{\frac{1}{2}}(x^2-ax+4a)$在$2,+\infty )$单调递减，则a的取值范围（    ）