[单选题]
17.17: The following lengths are ad bgvgq:,7 +jnuded together ) i(-lcs8h furd0+por: $L_1 = 8.36 \times 10^1 \; m$, $L_2 = 9.20 \times 10^2 \; m$, $L_3 = 1.39 \times 10^{−1} \; m$, and $L_4 = 2.1 \times 10^0 \; m$. How many significant digits are in the resulting sum?
