Which of the following choices is the explicit formula for the following geometric sequence?

-0.1, 0.02, -0.004, 0.0008, -0.00016

a. an = 0.1(0.2)n
b. an = 0.1(-0.2)n-1
c. an = -0.1(-0.2)n-1
d. an = 0.2(-0.2)n-1
e. an = -0.1(0.2)n-1