A reaction follows the given concentration (M)− time graph. The rate for this reaction at 20 seconds will be: (A) 4 X 10^(-3) (B) 6 X 10^(-3) (C) 7 X 10^(-3) (D) 8 X 10^(-2)

A reaction follows the given concentration (M)− time graph. The rate for this reaction at 20 seconds will be:
(A) 4 X 10^(-3)
(B) 6 X 10^(-3)
(C) 7 X 10^(-3)
(D) 8 X 10^(-2)

Solution=
Draw a slope for 20 seconds
slope =−dy/dx=−dc/dt=−(0.3−0)/(40−0)  ≈7×10−3