Which expression is equivalent to -2.1 + (-5.9) + (-3.7)?

A. (2.1 + 5.9 + 3.7)
B. (2.1 + 5.9) + (-3.7)
C. -2.1 - (5.9 + 3.7)
D. -(2.1 - 5.9) + (-3.7)

Please explain so I can understand.