Compute each of the following mentally and describe an approach that is valid:

a. 525 + 44 + 475
b. 396 - 97
c. 220 + 220 + 60 + 80 + 40 + 99
Which of the following approaches is valid for solving these calculations?

A. Adding each number together individually.
B. Subtracting the second number from the first number.
C. Adding each number together in groups.
D. Subtracting the last number from the sum of the previous numbers.