The table shows the concentration of a reactant in the reaction mixture over a period of time.

reactant concentration
time concentration
0 s 1.8 m
210 s 1.2 m
450 s 0.8 m
580 s 0.6 m
720 s 0.4 m

what is the average rate of the reaction over the first 450 seconds?
1.7 × 10−3
1.9 × 10−3
2.0 × 10−3
2.2 × 10−3