The dscussion of greenhouse effects have usually focused on the possibility of Earth getting warmer and the extent to which it might,-->我认为it指代不明;将定语从句还原是 it might the extent to没有谓语,且不知道是什么,虽然看似比较的是the possibility和the extent但是从句义上面说不通
the extent 在B选项中指的是一个特定名词,可以是任何的事物的extent; 而C选项中的to what extent 只与 grow to 相对应,即 what extent Earth would grow warmer to。
这一题B选项最主要排除的原因在于前半句 “has had as its focus”,因为focus有动词形式,直接使用动词会比名词形式更为直接明了。