Select the correct answer.
 Which rock has the highest metamorphic grade?
 A. 
 rock formed at 500°C and 8 kilobars of pressure
 B. 
 rock formed at 1,200°C and 20 kilobars of pressure
 C. 
 rock formed at 400°C and 12 kilobars of pressure
 D. 
 rock formed at 700°C and 10 kilobars of pressure