可惜lz一直没等到答案,我当时选时也有E做迷惑,但还是选了c,查了下有人是这么说的 The reason I chose C over E is because E says that 'Leaching had been a method...' who knows it may still be a method!
While the original sentence is trying to say that 'it had been well established' and does not convey anything of the current relevance of the method. 可以理解,但似乎说服力不是特别强,