Which equations illustrate the commutative property of addition?

choose exactly two answers that are correct.

a.
–5 + [6 + (–4 + 3.1)] = – 5 + [(–4 + 3.1) + 6]

c.
–7.4 + (13 + 2.1) = (–7.4 + 13) + 21

d.
–9 + 11 = 11 + (–9)