In a reaction, a chemist is observing the absorbance of CoCl2(aq) as a function of time to determine the rate at which it is being consumed?

In a reaction, a chemist is observing the absorbance of CoCl2(aq) as a function of time to determine the rate at which it is being consumed in a reaction. At time t=10 s, the chemist determines that the concentration of CoCl2(aq) is 0.12 M. At time t=20 s, the chemist determines that the concentration of the CoCl2(aq) is 0.090 M. What is the average rate of the consumption of CoCl2 over this time period?

a.
0.30 M/s

b.
2.1 M/s

c.
300 M/s

d.
0.003 M/s

e.
0.021 M/s