What is the recursive rule for this geometric sequence? 1/2,-2,8, -32, ...

A. a1 = - 4; an = 1/2 × an-1
B. a1 = 1/2; an = -4 × an-1
C. a1 = 1/2; an = 2 × an-1
D. a1 = -1/2; an = 4 × an-1​