What property is shown by the equation?

column a

column b

abc

1.

(34 + 11) + 9 = (11 + 34) + 9

abc

2.

25 + (–15 + 4) = (25 + (–15)) + 4

abc

3.

–(–21 + 8) = 21 + (–8)

abc

4.

–30 + (–16) = –16 + (–30)

a.

commutative property

b.

associative property

c.

opposite of a sum property