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Art historians are using a process known as infrared scanning in analyzing the Mona Lisa to determine if it has been altered since completion and if Leonardo da Vinci first sketched the figure in black, as done by many artists of the time.

(A) if it has been altered since completion and if Leonardo da Vinci first sketched the figure in black, as done

(B) if it had been altered since completion and if Leonardo da Vinci first sketched the figure in black, a practice employed

(C) whether it has been altered since completion and whether Leonardo da Vinci first sketched the figure in black, a practice employed

(D) whether it was altered since completion and whether Leonardo da Vinci first sketched the figure in black, as was done（C）


(E) whether it had been altered since completion and whether Leonardo da Vinci first sketched the figure in black, a practice done


请问C为什么是现在完成时，画完成已经是过去时了，alter     发生在完成前还是完成后？E的过去完成时为什么不对？谢谢！

a practice done 和a practice employed 有区别么？

同问