Which expression is equal to $5.1 \times 10^{-3}$?

A$2.9 \times 10^{-3} + 1.2 \times 10^{-3}$

B$3.5 \times 10^3 + 1.6 \times 10^{-6}$

C$3.9 \times 10^{-3} + 1.2 \times 10^{-3}$

D$2.2 \times 10^{-1} + 2.9 \times 10^{-2}$