When simplifying the expression 24i- 12+36i, which step below applies the commutative property of addition?
A: (24i - 12) + 36i
B: 24i + 36i- 12
C: 24i + (-12 + 36i)
D: 12(2i - 1 +3i)

15. Use the associative property of addition to simplify 5.2 - (4.1 - 42i).

A: 9.3 + 42i
B: 1.1 +421
C: 1.1 - 42i
D: 9.3 - 42i