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.
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
E. whether it had been altered since completion and whether Leonardo da Vincifirst sketched the figure in black, a practice done

I chose C, which is correct, but I also thought D would be okay..