A teacher gives a test an gets normally distributed results with a mean of 50 and a standard deviation of 10. Grades are to be assigned according to the following scheme. Find the numerical limits for each letter grade.

A: top 10%

B: scores above the bottom 70% and below the top 10%

C: scores above the bottom 30% and below the top 30%

D: scores above the bottom 10% and below the top 70%

E: bottom 10%